So, pretty much materia that can be added to 2h weapons will hopefully be more "potent" than those that can be added to 1h or offhand. For example, using arbitrary numbers, materia for a 2h weapon adds +10 [stat] whereas materia for a 1h or offhand adds +5 [stat]; which in the end would total up to +10 [stat] if put on both the 1h and offhand.

Of course, those who use 1h/oh combo will have one more equippable item to "mix-n-match" which materia they will add - works for me. Especially if you can envision them adding other special types of materia that would give, say, "Enhances Shield Block", or "Increase Chance to block", or "Enhance this [Trait]/[Ability]", etc. etc.