Use both high beams and low beams at the same time?

You can drive the low beam relay from the high beam relay using a diode.

I would use a small diode to go from the coil supply on the high beam relay to the coil supply on the low beam - this only requires that the current increases by the amount to energize the coil - combined that will be less than 1A.

If you do this on the output side of the relay then you will have the low beam relay supplying all 4 bulbs ie about 20A instead of 10A...

Do make sure that the relays are supplied separately, otherwise you have the same current problem on the supply side. If that is the case then I would run new fused supplies to the relays.