First of all, can I use a Unidisk card and two Unidisk drives?
Yes, you can use any Apple IIc ard for a II+ or IIe in your IIgs and expect them to work flawlessly. (OK 99.8%)
Where exactly is this in control panel?
The control panel is a menu program built into the IIgs to allow you configure the built-in periferals. (Periferals On/off, CPU speed,clock, etc.) You activate it by pressing OpenApple-Control-ESC. If this does not work from the program you are in, hold doen the option key while bootin. (Or is it the open apple key?...)
After this I can boot from 5.25" drives using my original disk card and from the 3.5" drives if nothing is present in the 5.25's?
Yes, in the slots, you can set each periferal on or off (Built in functions vs 'Your Card') and at the bottom of the list you can tell the IIgs what slot to boot form. Keep in mind that generally is will look at the slot and move on down sequentially to slot one looking for a boot device.
Most of the time, I press Control-Reset and then PR#x, where X is the slot I want to boot from. In more organized days, I'd boot to the HD and have a simple BASIC program that allows me to reboot to any slot or load GSOS. :)
Thankx, Ed