At the moment it is not possible to host multiple Mx projects in one single GIT repository.
If you are trying to do that you will loose the VersionControl integration inside Mendix Studio Pro and the possibility to Merge the code. (out-greyed)
MxSP expects that the Mx project is located in the root of the GIT repo. (The support confiirmed that)
Why could it be helpful to manage multiple Mx projects in one repo?
If a Mx application would be divided into multiple microservice the management in one single GIT repo would make the handling, merging and maintaining much easier. Without that integration the administration overhead is much higher then with such kind of funtionality