Never tried to model while rendering but you can have more than one blender instance running at the same time.

I don't know if you can model and render at the same time in the same blender instance, sounds dangerous to me. I remember I once did something like this and the result was a corrupt render because changes were applied to final render while I was editing. That's the reason I told you could launch more than one blender instance, meaning you can open blender twice and render in one and do something else in the other.
As has been said you need a powerful machine to do this or multiple computers. If using a single computer you have to share resources between tasks (CPU, GPU, Memory, HD I/O). Cycles uses a lot of CPU or GPU to render, if you use GPU you can use CPU for modeling on another window if you use CPU you have to limit the number of threads used by cycles. And be sure to have a lot of memory.

I usually leave the computer rendering at night all alone or use one of my other smaller computers while rendering. Render is very fast in today's computers, I remember when I started using computers simple renders could take two or three days to complete.