Simon Brown has some excellent commentary on the idea of “Technical Project Managers”:

In essence, teams get in a Technical Project Manager because they think people in this role can successfully undertake the project management *and* architecture roles on the project. Unfortunately, my experience suggests that this isn't always the most successful approach.

I’ve seen this several times over the years – one can do the Project Management, or one can do Architecture, but almost without fail it seems impossible to do both at the same time on a reasonably sized project.  One role will be more emphasised than the other, at the detriment of the background role.

Both these roles are of such importance that crossing concerns is asking for trouble.  At a pinch, I would recommend using part time resources for each role, rather than one resource trying to cover both roles.