Comparing joint to separate "what if" is not very accurate. The best and accurate way is to use the desktop version and create three .tax files. Two for separate returns and one joint return. There is no way to combine returns or tax files. Doing that online would require three test accounts.
There are just too many variables and the software would need to be very complex to do that automatically for a feature that most people do not need. More complex software adds to the cost and time to apply updates increases. Income allocation for filing separate in community property states is very complex, depends on state law and individual circumstances.
These features have been asked for many times in the past but are not cost effective to develop for a very small market - that is a marketing decision.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
