In theory, could a spell have two subschools?

[qwertyu63]

The title says it all. I'm making up a spell that would fit into both the Healing and Summoning subschools.

[Agincourt]

I'm having difficulty thinking of a reason why not. If the Player's Handbook II can put some spells in 2 schools, I don't see why you cannot put a spell in 2 subschools. An overarching theory is lacking for why spells are in the schools they are in.

[nedz]

There are some spells which could be assigned to multiple schools. In principle you could come up with arguments to assign almost any spell to all eight schools: since the arguments about which school a spell is in are largely fluff. It just depends upon how creative you are with reality.

So the answer to your question is Yes.

[Psyren]

The title says it all. I'm making up a spell that would fit into both the Healing and Summoning subschools.

There are spells like that; for instance, Gate is part of both [Calling] and [Creation].