Returning to this discussion, I had the thought of basically enshrining the concept of "level dips" as a house rule.
Firstly, figure out what 4E-style class category any given PF class would belong to: Leader, Striker, Defender, Controller. (We will not be using power sources.)
Then, at 5th level and every 5 character levels thereafter (10th, 15th, 20th), a PC or NPC gets 1 Level Dip point. Level Dip points accumulate level by level until spent.
One Level Dip point can be spent to gain a free level in a secondary class of the same class category as one's primary class, and this free level does not count as a level for purposes of advancing by XP (though it contributes character level in all other respects). For example, a Rogue 5 can gain a free level in Ranger or perhaps Barbarian. Classes of the same category will synergize more effectively than classes filling different party roles (fighter vs. wizard, for instance).
Two Level Dip points can be spent to gain a free level in a secondary class of a different class category from one's primary class, for example Rogue 10 / Druid 1.
Additional levels can be acquired in the normal fashion (costing XP), and level dips can only be spent on classes which the character has at least two fewer levels than their highest-level (or primary) class/es.