I Agree Stereo is a "must".

Stereo uses 2 channels that each have a different signal. Most small band PA's use a single channel amp and 2 speakers. This is mono since the signal to both speakers is identical.

To play in stereo your band will need a stereo pa with 2 seperate amplified channels.
On the mixer, you pan the left input of your keyboard all the way to the left and the right input to the right. If you don't do this the mixer will divide both signals over both channels and the stereo effect is gone.

Your Roland Amps have stereo inputs.
Reason for this is to connect a stereo keyboard to 1 amp without losing any signal.
The effect is a combined mono sound of both your inputs.
If you use 2 amps, then you connect your left signal to one amp and your right to the other. Now you have stereo again.

Good luck

Arno Thijssen