In my own games, I allow caster levels of the type of magic to stack. for instance Arcane with Arcane, Psionic with Psionic and Divine with Divine. By caster level, I am reffering to actual caster level, not the +1 level of existing spellcasting class. Basically the spells they already have become more powerful, but they wouldn't gain anything new from the class they weren't taking that level.
In addition I allow a two feats that work within this system. One allows caster types of different types of magic to stack, ie Arcane with Divine and vice versus. The other gives a class with no caster level a caster level equal to half their level in a particular type of magic. Ie a fighter 10 could have a divine caster level of 5. In addition a character that already has a caster level at half their level would be bumped up to full caster level (ie Paladins and Ranger, since the bard has full caster levels already). This feat can also be taken more than once to add caster levels to new classes or to bump up the ones you already have.
In addition I allow a two feats that work within this system. One allows caster types of different types of magic to stack, ie Arcane with Divine and vice versus. The other gives a class with no caster level a caster level equal to half their level in a particular type of magic. Ie a fighter 10 could have a divine caster level of 5. In addition a character that already has a caster level at half their level would be bumped up to full caster level (ie Paladins and Ranger, since the bard has full caster levels already). This feat can also be taken more than once to add caster levels to new classes or to bump up the ones you already have.