If you have 350W as it stands, and 500W at 2ohms, you will probably only get ~400W @ 2.67ohms which isn't much of an increase.
The extra speaker will push some more air though and if you stack the combo and 1X12 cab, the speakers will be nearer ear level so you may like that improvement - I would try it out before parting with any cash.
If you can, look out for a 2X12 @ 4ohm - they are far more common and you will definitely get an improvement.