If you are putting them in series to get a voltage output of the two individual cells it would work, but it is not good practice.
Putting them in parallel is a no, no.
I take it you are not racing the car - Lead acid cells are heavy for the power output capability.
Ok, I think slight misunderstanding, which is my fault for not explaing clearly.
When i say mixing, I don't mean combining the two batteries together. My RC car came with a lead acid battery which is now dead. I want to know if i could buy a similarly rated Ni-Cd replacement instead of a lead acid. Or will the circuitry be designed for lead acid.
The main reason for this is that the lead acid battery supplied with the car is 10V, and I can't find any 10V replacements.