Question

can we connect two voltage sourses in series or parellel combination?

Answer 1

Of course.

Batteries.

In series, the emfs (E = E1 + E2) and also internal resistances are simply added while in parallel, use E = E1R1 + E2R2/(R1 + R2)

where R1 and R2 are internal resistances.

Answer 2

And of course when parallel, the current capacity has doubled.