I think Thompson is the starting center, when he is ready. I think that is pretty easy.
To me the question is whether or not Theis can fit in as a second starting big (or core rotation big) and play alongside Thompson. PF is probably a more natural position for him. In 2018-19 Theis hit 39% of his 3s. If he can do that, and adapt his offensive game around that, more perimeter, he might just find a nice role for himself.
Theis can let Thompson be the more "blue collar" guy, defend the bigger, stronger player, focus more on protecting the rim and rebounding, and Theis can adjust his game and try to be more like Kevin Love but for a lot less money (Love is career 37% on 3s, just takes way more). If he can do that, it will help the team.