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Should work OK. Keep in mind that on most dual-SIM phones, the 2nd slot is often GSM-only (2G). If your intent is to be able to get your SMS, should be fine.
Also, keep in mind that T-Mobile's roaming in India can best be described as a mess. For instance in some areas of J&K State, there is no roaming, even though it's served by the same carriers as the rest of India. If you're headed that way, it might be good to have a phone that supports T-Mobile's WiFi Calling.
Hey there, do you know what type of phone you're thinking of purchasing? My biggest concern would be compatibility of a foreign phone with the T-Mobile SIM. There are some pretty outstanding folks on these boards who can break that down to a science if you let us know what phone you have in mind.
As far as recommendations go, the OnePlus a series of dual SIM phones that seem to have a fair track record with T-Mobile service, and they are sold unlocked for international use.
Unlocked Dual-SIM phones with TMO SIM in one of the slots should work fine.
Traveling overseas (with most Simple Choice plans to my knowledge), TMO data speeds would be at 2G (up to 128 kbps) speeds anyway.
It probably really would not matter which SIM slot the TMO SIM is placed in, as far as overseas service per se is concerned but,
for ideal local service coverage I would think the local SIM should be in the primary slot.
Caveat of course being, which region is the Dual-SIM phone primarily designed for.
Info on TMOSimple Choice™ plans
Another page worth going over is International roaming checklist
I see a lot of answers here that are good, I just wanted to provide a bit more information. In the US we have 4 major wireless providers and a handful of smaller regional players. Of the four major providers Verizon Wireless and Sprint use CDMA and will probably not be compatible.