I would be supprised if it would be reliable. What usually happens is the excess load causes the dip switch to weld together.
I woud use the mail beam to drive a rely with enough rating to carry the current of the spotlights and run a direct fused line from the battery.
I am no Sparky but I did what you are wanting to do to my 1275GT Mini many moons ago and ended up with a burnt out dip switch and no lights. I was then given a diagram wiring up Dip Main beam and the spotlights using relays so I had normal dip or mainbeam, dip and spots.
