Regarding the main point of the thread, I think it goes against the idea behind multi-melding. I mean, combining materia or providing more slots for melding is exactly the same thing in the end.

That being said, I could see that the rates of success for adding an additional materia could be considerably higher if added materia is from a completely different category, like you mention.

Instead of dropping to 25% success on 2nd damage-related materia (say 1 +str materia and 1 +attack materia), you could drop to 75% success if 1st materia is damage related and 2nd is resistance related.

This would promote balance in the melded-gear rather than stacking up only the most obvious materia at a high risk rate.