You've probably got *just* enough lift in your BC. That being said, I don't think that you'll find it particualry comfortable, and it may stop you from progressing your diving further.
I have twin 12's and twin 15's, and to be honest the 15's are a git out of the water, and are only a tool for the job.
If you've decided that twin set is the way to go for you, I'd seriously consider buying a new wing and twin 12's. You could probaly pay for most of this my selling your current BC and 15's on eBay
Cheers
Rich