I downloaded the form from the MN Dept of Revenue and started filling it out manually just to see how it would go. Turns out there are maximum income caps on the marriage credit. That is, if the income of one spouse is over a certain threshold (which varies depending on the other spouse's income), then the credit is zero. Of course the state of Minnesota doesn't say that part too loudly, and in the requirements no income criteria is listed. It's not until you get the to the actual table and look up the income values that you figure out the credit is zero. TT does not do a good job of communicating this in the program, and of course the state likely intentionally tacit on this fact.
Anyway, hope this can help someone else out as I have not seen this credit described in detail elsewhere on the interwebs.
