If you wire additional bulbs to your turn signal circuit, you may find the turn signals flash too rapidly due to the extra current from the additional bulbs.

Using LED lights instead of incandescent bulbs may help, or you could install an electronic flasher unit instead of the mechanical one, and not worry about it.

There's no reason why you can't install a dual filament bulb in your side marker lamp assembly, assuming it will physically fit, but you will have to change the lamp socket to accommodate the bulb. It might be easier to just mount a second bulb/socket in the side marker housing and wire it to your parking lights or turn signals.

Bill