Not sure it would make a difference as the T-Mobile version is single sim only.
I don't own the 6T but I have very seriously looked at buying one and seen many many articles and you tube reviews on it. the TMO version only has single sim support. in one youtube video a reviewer showed the sim tray, one part was solid plastic where there probably should have been a cut out for a second sim. weather that means just the tray is different or the internals of the sim slot are different as well I can't say but I did notice that one difference.
if dual sim is a thing you must have I would suggest buying the unlocked version from oneplus directly, that means no trade in deal or financing options with t-mobile, Oneplus accepts paypal and paypal offers credit. if you use paypal credit you an finance it for 6 month no interest.
Hey there! I am very sorry but flashing roms is not something we support as the carrier. There are a few Android experts here in our community but these types of inquiries usually gain more traffic on Android based forums.