So I'm about half way through the game and have maxed one-handed. Would it be safe to start training the two-handed skill or will this break the game in some way?

One handed will be used for any boss battles etc and two-handed just to clear out easier dungeons etc.

No it will be very dangerous to do so. Your character may forget that he is using a 2H sword, try to swing it one handed and break his wrist horribly, rendering him unable to swing a sword ever again, and forcing to to turn into a milk-drinking magic user...

no worries , you won't break the game .

Enemies are levelled to your level which assumes a certain offensiveskill which you might lack when going two-handed that can make the game harder. Other then that go ahead.

if you have enchanting pretty high, you can just enchant urself some gear to fortify 2-handed and your set to go!

I don't see it as breaking anything. That's more of a playstyle issue. Even if you got your 2H to 100, you would still kill the enemy just as efficiently. I would go with one or the other. But it is your game. As long as you are having fun, that is all that matters.

I'm gonna do that soon too...to get to higher levels otherwise imposible. I gues if we won't switch both weapon and armor skill at same time we should be fine. Also enchancements for the new skill is something that I'll take for sure.
Or you can use (posibly exploit):

I went the other way, 100 in two handed. One hit kills were getting boring. I'm now going one handed. It's not a problem but I keep the 2H ax handy, some encounters are deadly.

Not really likely to be a problem. Areas are keyed to your level the first time you go there and stay that level forever for that character, plus certain (most?) areas have limits to how low- or high-level they can be, so you're not going to be seeing foes of your own level all the time, everywhere in the world, like you did in Oblivion. My level 70 Khajiit still meets level 1 bandits when he passes by Riverwood.

So keep your best weapon handy in case you need it, but you should be fine using a new weapon type to cross-train when you run into lesser enemies.

^{it's a odd combo of wepons but it's your game, if you use it sure go for it}

If you want to do this, USE the ebony blade (the daedrica artifact) to raise you 2H skill because its damage is calculated using the 1H perk tree instead. Also, its swing speed is the same of a 1H blade thus the DPS factor is a bit higher making it perfect for beginners.

Thanks for the advice all, much appreciated (y).

complete waste of perks. level block or armor or anything else that makes sense!
Even speech!

Level 1 bandits near Riverwood when your character could be lv 50+? I'm just glad that I didn't go crazy with the exploring early on, as my character would have no challenge in these areas as a high level character.

Skill more or less does not matter really it comes down to Perks. Your effecitveness is dictated by the Perk investments you have made in each category and further modified by the Weapon and how improved it is. "Leveling" a skill is very very easy to do.

Umm, he didn't say he was going to perk two handed, only that he was going to "train" it, which I assume means either pay a trainer or use the two handed weapons in battler. Either way, it will increase his level more than not using it, which will lead to more perks that he can use on whatever he wants.

You decide to train or use any skill whenever you choose. I'd prefer to do that next time so maybe I won't be a beast on master.

The catch is that late in the game, leveling is slow and perks rare. And 2H (like just about every skill) needs at least four perks to be any real use. It's one reason I'm not a fan of the current leveling and perk system.

heh, i got 1H to 100, now im leveling ranged in ebony armor...
yeah, i need glass armor...

Yeah, I didn't say that levelling 2H as a secondary melee skill would be all that useful as a combat strategy. I am not that big of a fan of Skyrim's leveling/perk system either, but it won't break the OP's game if he picks up a 2H weapon and uses it or pays to trian in it. I think that is what the OP was asking, whether the level gain by raising his 2H skill would level up opponents beyond his power to deal with them. The answer to that is "no" because you will get enough health/stamina/magicka and perks on level up to compensate for the increased opponent strength, provided you spend those perks in the right place.

Dude I've leveled one-handed,2 handed, light armor and heavy armor and have never put a perk point in speech or block and my guy is doing just fine on expert. What "makes sense" is whatever the OP wants for his character. This game aint that hard where if you use a few perk points to train something else youre gimped...damn you get up to 81 perks if you so choose, so its not a problem.

My primary is a sort of chameleon. Anything for fun - just don't ask him to actually run a guild or do anything else that resembles that nasty four letter word (work). His inclination is the TG, so that is done in his true character - except he will leave rather than be guild master. He did the DB quest line as a vampire. Got cured, sort of regrets it, but not really too much.

Which brings me to this topic. He is going to do the companions quest line, but to him it is more of a lark than a real commitment. He will show up in all ebony with to handed weapons and do the quests just for the fun of it. Once he is done he will walk away from it all much like he does with everything else. From a game play PoV he gets to use heavy armor and2 handers for a bit, and he gets to do so while still staying in character.

I'm currently playing a character that uses both 1h and 2h concurrently.

I do this for RP purposes and its working rather nicely.

As to your issue of having a skill at 100 and leveling the other, I see no reason why not.

There are a handfull of two handed weapons that use the one handed stats but register as two handed when increasing skills, like the Headman's Axe or Ebony Blade for example.
These you can use to level up your two handed without worrying about being underpowered in a fight.

Go for it, just keep carrying a decent one-handed weapon in case you come across an especially tough enemy you can't beat two-handed.