There's a few threads that address this. In a series installation you usually have on pump that pushes water through both boilers. Disadvantage is slower response and more heat loss. In a parallel configuration, each heat source has its own pump with check valve and they're connected to a common supply and return. See the plumbing diagram on my site for an example - link in my signature below. You only care about the left side of the diagram - the right side has complexities unrelated to your question.