There is no good way to do the comparison in TurboTax Online. You would have to have two additional separate accounts to create the two married-filing-separately returns, and you would have to create each one from scratch. In the CD/Download Desktop TurboTax software you can use the What-If Worksheet to do an estimate of how separate returns would work out compared to a joint return, using the information from your joint return.
Don't spend a lot of time or effort figuring out a way to do the comparison. It's extremely unlikely that filing separately would come out better.
