Some answers, posed as questions:
1) What if the two source presets require different blocks and there's no room on the grid?
2) What if the two source presets both want control over a certain block on a certain channel (Ex: "Amp 1A")
3) What if the two source presets require different architectures (e.g. blocks in Parallel vs. Series)
4) What if the CPU can't handle the combined blocks needed for both source presets?
5) What if the two source presets have different modifiers on the same parameter?
6) What if the two source presets use inputs and outputs differently (potential for hearing damage on this one!)
7) What if the two source presets use scene controllers differently?
8) What if the two source presets use the same blocks but in different orders?
9) What if the two source presets each depend on the same FC resources?
10) What if we solve all these problems just in time for
the distant future to have arrived? ("Grrr" factor)
These problems multiply as you realize that more up to EIGHT source presets could be in play.