If i wear it to gain 2 standing stones, and take it off... what happens? Do i keep the effects of both standing stones or only the effects of the former/latter?

Nope, you get only the newest effect.

I'm still confused with the Crown, lets say I picked Thief and Warrior and then got the Crown
Would It keep those two?

This makes no sense. Until you get the Crown, you can only pick one standing stone - so you'll have one. Then you get the Crown and you can pick another one.

The easiest way to understand it is to think of it as the Crown emulates a Standing Stone bonus as an enchantment.

when I unequip, then re-equip the crown, I get both effects back. only the newest stone stays with me when it is off, though.

I was wondering if the effects were cumulative, as well. I was thinking of grinding with Lover's Stone + a random Guardian Stone.

That's the only way I'm hitting 70+ for the damned Legendary Dragon achievement. Either that, or start a new character and glitch my way to 81 with the Ohgma.

I have a question. The crown isn't even working when I pick a new standing stone. It just changes my current one but the crown doesn't take the effect of the old one. Any help?

This is worth some personal testing.
I currently have the crown in my end table, not even in my inventory, and the two chosen stones are shown as active on my Active page.

1. Initially it would remove the previous stone when unequipped and the Active page would reflect this.
2. So I unequipped the crown, chose a third unwanted stone, then went and chose what would be my secondary stone.
3. I then travelled to my primary stone, chose the stone, then equipped the crown.
4. After unequipping the crown, both my desired primary/secondary stones remain active.
5. I stored the crown in the end table in my Alchemist's Shack and my Active Effects tab clearly shows both Lord and Warrior as active.
6. They haven't changed in well over 30+ hours of additional gameplay.

We don't yet clearly know if you need the crown equipped or just to choose your stones.
Whether glitch or intended gameplay, my current Dawnguard character has two active Standing Stones and is wearing a helm, not the crown.

Time will tell but, for now, perhaps you can try the above steps for yourself and see what happens.
Hope that helps

So the enchantment basically acts as a standing stone, and when you select a stone, the previous one becomes the enchantment on the crown?

Removing the crown takes away the effect the same was as taking off an enchanted ring removes the effect of the ring. But putting the crown back on, you get the standing stone bonus back and don't have to go to the same stone over and over again every time you take off the crown? Good to know.

Last thing I'm curious about is the Lovers stone + Guardian stone. If I chose, say, lovers stone and mage stone, would I get +15 sneak and combat skills then +35 mage skills?

I think so. What I'm curious about is whether or not Rested-Lover's Comfort take effect if the Lover's Stone is isolated to the Aetherial Crown/you unequip it to gain the bonus and then requip it. That would give you 30% increase on all skills plus another 20% to whichever stone you have activated right?

They could have made it much easier.

With no crown activating a stone has the same normal effect. When the crown is worn, activating a stone brings up an options menu:
- activate stone for character
- activate stone for crown

Easy.

That is genius. I still have a difficult understanding on how to use the crown myself. Is it when the new standing stone effect is added to the crown while it is worn but doesn't effect you once you remove it? Still confused about it.

You could just get the Lover's comfort active effect by sleeping in a house with your spouse

I just adds the last stone you had before your current stone...
If I get the Lord stone, then go get the Lady stone, then equip the crown, it will add the Lord stone.

Basically, what the crown does is, the first time you put it on- get this -NOTHING HAPPENS. You keep your normal one, and the last one you had just isn't their. You have to go pick another one, then the one you first had basically, as others have said, the enchantment on the crown. Then, if you take it off, you have the more recent one. then if you put it on, it actually does remember the last one.