You can earth the side and spot together, that's fine.
I'd go with the idea of taking the power for the spot via a relay so as to not put too much load on the existing wiring.
Earth the lamp/bulbs.
One terminal of the relay goes to the earth as well.
One terminal of the relay goes to a positive supply (either permanent from the battery (with fuse in line) or via a control relay already ion the vehicle (either accessory or ignition on relay), this way you control whether they should always come on with high beam or only with high beam if the accessories (or engine) is on.
One terminal goes to the existing wire to one of the high beam headlights.
The remaining terminal goes to the +12v side of the lamp/bulbs.
Haven't got a relay to hand to give you pin numbers, but the earth and existing high beam go to the actuation solenoid in the relay (symbol is a rectangle with a line coming out of one end.
The +12v supply (battery/ accessories supply/ engine supply) and the +12v to the lamps go to the pins that look like a fence with a gate in it when seen from above _/_ sort of symbol.
I'm sure it will be clearer once somebody posts a wiring diagram for you.
Andy