You didn't read what I said. I wasn't talking about AT&T roaming to T-Mobile, I was talking about AT&T roaming to its local/regional partners.
1. Prepaid AT&T MVNO carriers do *NOT* roam on local/regional carriers, so they won't work on those 'tiny ski towns' or 'off the beaten path' either (where you see that AT&T have coverage, they're really roaming to regional carriers). Whatever advantage you think you're going to get with an MVNO compared to what you get right now with T-Mobile, it would be minimal, if there's anything at all.
2. Have you looked at the current AT&T postpaid plans? Postpaid plans have changed a lot in the past year and they're getting much better. Especially if you need a lot of data and you need AT&T coverage, you need to be on postpaid. MVNOs data are expensive, and again, they don't roam.
3. If you rely your decision solely on those coverage maps (without actually testing the coverage with an actual MVNO SIM card), you're setting yourself up for disappointment.
To be absolutely clear: you seem to want an additional SIM card from an AT&T MVNO that you can switch to when you have no T-Mobile coverage. The idea sounds pretty good, but it won't work because the likelihood is when you don't have T-Mobile coverage (especially because you said these are rural area, off the beaten path, etc.), AT&T MVNOs won't have coverage either. You might as well ditch the two-SIM-card plan and go with postpaid AT&T (which will give you coverage in these cases, and you only have to pay for one line).