When looking at something very similar, I looked at the ability to independently deploy each unit.
If each unit can be deployed completely separately, even living on separate servers, then it may make sense to have them as separate projects. You can build and deploy each unit completely independently when there are bugs that need to be fixed. This can provide some benefits if you wanted to have separate SLA's or release cycles.
If each unit is very dependent on each other to the point you cannot deploy them as separate units, a change in one requires a release on the other, then having them as separate projects will complicate the deployment process with out an associated gain.
Like a lot of things in tech the answer is "it depends" but looking at the way you are going to deploy things can help determine the right answer.