The more i think about it, i think the solution is very simple...

Make arrangers that allow you to add a seperate midi lower channel..
Like it works on the Wersi Abacus and pegasus wing arrangers..
This way, everyone can choose to add a lower manuall and pedals..
Its also possible to add a lower manuall on the Tyros5...
But controlling/switching the sounds in real time is near to impossible...

I dont think its possible to add a lower keybed to the pa4x and controll internal sounds...