Hi and Welcome!
This comes up periodically. I hadn't heard of Littermate syndrome til after I brought home my two littermate mini boy pups. I'd had previous experience with 2 littermate sisters but they were nearly 2 years old when we brought them home.
I have no experience with the training and behavioral differences between toys and minis so I defer to the toy owners there, but I can say two things about my experience. It will be far more work with two than you can imagine. It's as if you have a flock

. And, it can be done successfully - if you have the time to devote. DH and I are retired, so we had the time. It's one heckuva ride, and even with the inevitable rough patches, it was well worth it for my family.
See this thread
Help! We brought home littermates!
It's from a member who brought home spoos, but pooparents of all sizes chimed in. You can use the search function also to find other threads on this. Just search for Littermate syndrome.
Good luck! Whatever you decide, we'll be here for you

.