Alain,
You don't need another mixer because your amp has on built in. If you want to hear from the keyboard speakers, plug the secon keyboard right INTO the inputs on the back of the first keyboard. Then you plug the first kb into the amp (any channel) and you will hear both.
The Roland amp has a stereo mixer, but it is a MONO power amp, so unless you have a second amp - just use one mono connection coming out of the main kb. You will still hear the stereo imaging from the keyboard's internal speakers.