When wiring relays, why not use the floor Hi/Low dimmer switch to toggle the 2 relays. 12V Control lead comes out of the Master headlite switch for ON/OFF operation. Then once the headlite switch is ON then the floor High/Low switch will toggle the respective relays. No additional switches or cutting is required, also I would suggest that you consider to mount the relays under the battery tray as to keep the headlite wires as short as possible. Regarding the actual heavier wire for the headlites, suggest you use # 8AWG.
Be sure to solder the appropriate terminals and use shrink tubing.

Just my $0.02..