Quick Lipo matching Question

I ordered two 3s 4000hv maxamps packs. But since 6s is too much for my 8xl, and 3s is too little, can I do this:

Buy a 2s 4000hv maxamps pack, and run it in series with the 3s 4000hv pack, making it essentially a 5s2p 8000hv pack? Or will this result in a problem?

Also, how much runtime do you think I would get from a 8000mah Lipo?


Thanks!!