info on how battery work

hi looking for some info on this I have 2 4s 5000 mah and need to know if better on a single bat to run heat or 2 in paralelle .I know that in paralele you double mah and series is voltage what I need to know is
parallelle is more for power or longer time or not please let me know it just info looking for
thank you

Higher mah and higher "C" rate = higher voltage under load.
Seeing that motors (KV) X higher voltage = higher RPM

(C) rating - Watch out for cheap batteries with a high c-rate
GOOD batteries are not cheap.

Larry