I have a pair of Hella 500s mounted, but after numerous trips to Baja (dodging cows at night and god-knows whatelse) I want to mount 2 more for more lumens.

Can I run two relays off one switch or do I have to run 2 completely isolated systems?

Sure. You can run 2 relays off 1 switch.

Two relays off of one switch - no problem. Two sets of lights off of one relay - possibly - as long as the fuse, wire, and relay are rated for that much wattage you're alright.

There's a couple of ways to do this.

(Paul is right on both counts...)

If you're running 55W lights and you have a 30A relay, you'll be fine with just that one relay. Just add the lights (55WX4=220W, 220W/12V=18.3A so the 30A relay can handle it).

The other way (probably more preferable) is to put in the second relay. That way you have two sets of two lights. You can run all 4 off of 1 switch as you questioned, as long as the switch is on the signal wire for the relays, but I'd suggest two switches. With two switches you can use only two (either set) at times if you want. (There's other advantages too like the use of smaller guage wire to power them, ease of troubleshooting if/when you have any problems later, etc.) Also remember that if you have lights bigger than 55W, you'll have to go with two relays as most relays are rated at 30A. Anything brighter than 55W will overload a single relay if you have 4 lights (4X100W=400W, 400W/12=33.3A - more power than a 30A relay can handle).

So, depending on what you want to do, anything is possible.

Hope this helps...

-Gonzo