There are now two equal resistances in the circuit, and bearing in mind that a battery store is emptied wherever an electric current passes through a resistance (first met) you should focus on the resistances. These are equal, so the forces on the charges in these resistances will be equal. What else could they be? So the same brightness for each lamp: same push and pull on each charge; same number of charges passing through each resistor.
There is no difference: the bulbs are identical, so what other possibilities need we consider?
The bulbs are dimmer because adding the extra bulb increases the resistance and reduces the current everywhere in the whole circuit.
As a result fewer charges per second pass through each bulb and each charge depletes the chemical store of the battery by a smaller amount as it passes through the filament.