You probably could use the same relay providing it's capacity is not exceeded by the amount of current (amps) your lights draw, but you probably don't want to.
I think you want separate switches and relays. You may find that you'll want to use your bumper lights when you are on-road to augment your high beams. In most if not all states, you can't legally use windshield lights on-road. They have to be at or below the level of the headlights. So if had both sets on one switch/relay, you would not be able to legally use your lights on-road. If they're switched separately, you can just throw on the bumper lights when you're on-road.
You might want to consider using a feed from your high beams, routed through a switch, to power the relay for the bumper lights. This way, they'll come on with your high beams, except when you have that switch set to "off".