It’s difficult to give you a definitive answer because there are many variables:

1. You need sufficient bandwidth and low latency in order to stream 2 4K streams simultaneously. The only way to find out is to try TMO Home Internet within that 14 day free trail period.
2. You mention that your TVs are Smart TVs. While this would indicate that streaming apps can run natively on your TV, you would have to refer to the TV maker’s manual or support to confirm that they offer the streaming app you are looking for and that they support 4K streaming.
3. The last variable has to do with wifi range. Unfortunately the TMO routers must be positioned within your house where it gets the best cell signal. This position (usually next to a window) may be pretty far from one or more of your TVs so again you can only confirm by using the 14 day free trial period.

Cali Cat is right on. It is pretty hard to provide a definitive answer regarding how the T-Mobile service will hold up. If you are in a really good location with a strong clean cellular signal and the tower is not over loaded with users then the chances are good it will perform pretty well. You can run speed testing with several testing sources and get a good idea of performance. SpeedTest.net, speed.cloudflare.com, fast.com or whatismyipaddress.com all have speed testing. I find testing against 2-3 to get an average is helpful. If your gateway provides a fast clean cellular delivery you should be ok. During peak hours maybe not as well as off times so it really depends upon load and the actual performance of the cellular signal delivery to the gateway.

Keep in mind different streaming services allow you to download movies for watching offline and this is actually a good way to go. It is difficult to know when service delivery will be impacted by higher user load. Peak hours in the evening are usually busy so just planning ahead with content downloads can lead to a much more satisfactory experience.