Yes, its very simple.

you need a on/on switch, this will have three terminals - usually in a line, normally you put power to the middle one and then the switch position determines which of the outer terminals the power goes to.

You're gonna use it in reverse - put the wire from the dip beam to one of the outer terminals, the wire from the main beam to the other outer terminal, and the centre terminal then feeds the relay. The switch position now determines which of the two inputs switches the relay.

You can also do this with a on/off/on switch if you want the additional 'off' position, wiring is the same.

hope that helps.
Pete.