I think a lot of people are too quick to place limits on their players just because they themselves wouldn't do something, or don't like it. The way I see it, if your players don't like the characters they're playing, they won't be your players for long. Or shouldn't, anyway, though I'm sure plenty of people stick with games they can't stand because it's the only group around. I know I used to, until I found RPoL.
Anyway, I personally like the idea of a multi-class feat, but I think Mr. Redrobe's idea is a little weak, given that both classes do indeed face spellcaster-like penalties for multiclassing.
How 'bout this?
[sblock=Dragonfire Shaman]Prerequisites
Draconic Aura +1, able to use at least one Draconic Invocation, and a breath weapon derived from the Totem Dragon class ability
Benefits
Your Dragon Shaman and Dragonfire Adept levels stack for the purpose of your breath weapon's damage dice and area affected, but only for the breath weapon derived from your Totem Dragon. Any other breath weapon you gain automatically deals the same type of damage as that provided by your Totem Dragon.
In addition, your Dragon Shaman and Dragonfire Adept levels stack for the purpose of the Caster Level of your Draconic Invocations*, and the bonus provided by your Draconic Arua ability.[/sblock]
* I'm assuming that, like the Warlock, the Dragonfire Adept's Invocations don't actually deal more damage or anything based on CL. I don't actually have the book, I've only read over the class once or twice.
This allows you to increase the damage for the breath weapon with the cooldown rate. You can still use your Dragonfire breath weapon while your Shaman one recovers, but the damage will be much less, which seems to be a nice balancing factor.
You gain more effective Draconic Auras by taking Dragonfire Adept levels, but you don't learn any new auras, which seems to work OK.