Prestige Classes that offer "+1 to existing (arcane/divine/either) spellcasting class" do not have their own spellcasting progression. They therefore are not eligible targets for other Prestige Classes that offer the same. No, you cannot be Wizard 3/Cleric 3/Mystic Theurge 1/Whatever X and continue dual-progression.

Would be interesting if you could. Worth it? Certainly. Overpowered? I'm not so sure...