Are there any "easy" ways to do this?

If I put the spell in one hand then I lose dual casting perks.
Right now it's also very mana intensive (level 3)
I haven't found a weapon to disenchant yet but would using a weapon be better than casting? I still see that mirroring my first issue.

Level conjuration and get a bound sword with the perk for bound swords to cast soul trap.

Soul trap is handy on weapons, but if you don't use them it's not a good solution.

You never cast it in the beginning of a fight so it shouldn't be hard to cast it in one hand when an enemy is near death then finish them with spell inn other hand.

Also consider what you're fighting and if you have a gem to hold the soul. No point using it on a human if you have no black gem.

It just takes a second to swap spells in your left hand and soultrap lasts long enough to dual cast after. As you say, a weapon has the same problem. The exception being a weapon with dual enchants for trap +damage but that requires 100 enchant and isn't that mage-like anyway.

My soul trip weapon has a 1 second duration, as long as it makes the killing blow it works.

Yeah, that works but many mage characters have low weapon skill and damage so it's too dangerous to be in melee range long enough to make a kill. It depends on how you've developed your character.

I understand. I guess my implied question was for situations in which I need to cast ST.

enchant any weapon with soul trap,,,bows work good for this, especially when you just need lots of soul gems, you can cruise arond killing deer or whatever, for bigger things when you need to just soul trap it...just go up to and soul trap it..you're gonna take a little damage but you need ot make sure you have some defense but one thing at a time..if a bear charges, i cast soul trap on it first...then just kill after..its not that hard...you're prob not gonna be able to soul trap and not take any damage...so just go with it...cast the soul trap then kill whatever it is...even if its a spider...i know theyr'e creepy but jusy stay focused..cast the soul trap and then kill it.

Yeah, this wasn't an issue for key thief character. Just trying to stay in the flow as a caster. Bows don't apply.

Then you need to beat an enemy as close to death as possible, then cast st and finish it off.