When using the online product, you would need to start 3 accounts. Prepare each married filing separately return in two of the accounts and the married filing jointly return in the 3rd account. Add up the results from the two separate returns and compare it with the joint return.
The CD/download product has a nice "what if" feature than will automatically do this. But it is not in the online product.
See the comparison of MFJ vs. MFS: https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...
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