To me, these tags are used almost interchangeably at the moment, and I think any attempt to separate them would be awkward. The tag wikis are almost circular as well, which is another indication that they overlap.
Wiki for mathematical-models:
Theoretical models that explain a system using mathematical equations.
Wiki for theoretical-biology:
The modeling of biological processes using the tools of applied mathematics.
My suggestion is to merge them, using theoretical-biology as the main tag, since this is a bit more general (could e.g. include non-formalized theoretical biology as well). I have already suggested this tag synonym, but not much has happened since the voting on tag synonyms is at a standstill.
I do see that there might/could be a slight difference in emphasis in these tags, where mathematical-models could be used in a more technical sense (any application using advanced mathematics) even if the focus isn't theoretical development of the subject. However, the current use do not match this and the number of questions that use these tags aren't that many. Therefore, I think it is more useful to merge them.
In either case, and after seing the answer from @Corvus, I have now tried to separate the two tags by editing their tag wikis:
"Questions relating to biological theory or theoretical developments that deals with biological processes."
full wiki entry
"Models that analyse biological systems or processes using the tools of applied mathematics."
full wiki entry
Please comment or edit if you don't find them suitable.