I might be better to not bother. Instead create a new class.
This is the route I would go as well. I am not a fan of multi-classing at all.
I am nothing, if not obliging to my players, and there has been some requests for moves along this line. I am also aware of at least one post that suggested multi-classing as a means of having a game with a single player.
When I launch the Golden City of Shanthopal, I want to be able to have different characters involved in different scenarios, sometimes even working in opposition to each other. I want a strong solo option to exist. Moreover, I think that "A class for anything and everything" was as much the downfall of 3e and 4e as the way 3e dealt with multi-classing.
DCC has very strong class identity, and I would rather allow more than one class than break that. "Fear no rule", though, and to each his own. I will be penning this as a potential house rule for those who are interested, with the intent that there are Faustian terms to the bargain.
In 3e at least, multi-classing traded what you might someday be able to do for a huge dose of versatility now. There really is little downside to such a deal. What I envision gives you the downside now, in exchange for a moderate dose of versatility later. Certainly, in discussions with my players, the multi-classing option I envisioned was thought a viable option, but certainly not ideal. No one would use it as the "obvious" way to gain power. Which is, IMHO, as it should be. It was extremely clear that, due to the cost, I would be seeing no characters with three classes!
It is also in my mind to limit demi-human classes based on their natures. I cannot see Poul Anderson's elves as Clerics in the DCC sense, and I do not think that should be an option. Again, anyone using my house rules who disagrees can adapt them to their own vision.
This may sound as though I do not appreciate the feedback, but I really do. Crom on His Mountain and Mog in His Web both know that I have no desire to recreate 3e out of DCC. Using single-classed characters should always be the most likely choice in this game. (But a multi-class option will also help my players convert surviving RCFG characters, and that's going to be important by the spring!)