In general, for resistors in a parallel circuit apply the formula...
1 / Ri = 1 / R1i + 1 / R2i + ... + 1 / Rni
Your circuit consists of two 9-volt battery with an internal resistance of 1.2 Ω...
1 / Ri = 1 / R1i + 1 / R2i
You can rewrite the formula to get...
Ri = ( R1i * R2i ) / ( R1i + R2i )
= ( 1.2 Ω * 1.2 Ω ) / ( 1.2 Ω + 1.2 Ω )
= 0.6 Ω
The internal resistance has become significantly smaller. This results in new values for the current and voltage in the load circuit, as you can see in the circuit shown.
In any circuit that is powered by a battery, the internal resistance should be negligible compared to the resistance of the circuit. A parallel circuit is one possibility to reduce the internal resistance.
But there are also other advantages...