I actually entered an amount on a "CO" line with the W2, then I was able to edit the MA part-year return non-MA amount. Admiteddly, that's not the easiest way to go about it, but it may be necessary. In addition, you are almost certainly going to need to mail-in the return with an explanation of the apportionment. But if you do it the way you suggest, as a full-year MA resident with a CO nonresident credit, I'm sure MA will find that acceptable and you probably don't have a large tax difference. I was away for two days so I apologize I didn't comment back sooner.
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