Question: When a headlight is specified as 12V 60/55, what is the 60? Is that the brightness?

Reason this came up is this: a friend's '80 GS750..he replaced the stock headlamp with dominators. His headlight fuse keeps blowing. Original specs say 12V 50/40W. If he replaced the stock ONE lamp with TWO lamps both higher wattage, would that cause the fuse to blow?

The 60 is the wattage, i.e 60 watts high-beam 55 watts low-beam.  Replacing 1 lamp with 2 of a higher wattage could definitely cause the fuse to blow, although given the age of the bike I would advise your friend to also check the headlamp wiring and make sure nothing is short-circuiting.

This is what I was typing as otown posted (amazingly similar):

The numbers are the wattage of the high/low beam bulbs (60/55 means 60W high and 55W low).  You definitely could blow the fuse by putting in larger bulbs, but it doesn't seem like you've moved up that far in wattage.  On second thought, TWO 60/55 bulbs would be a problem.

Check for a wiring issue (perhaps a short somewhere).

So you've got two votes for checking the wiring, and two for suspecting the bulbs.  Did you wire through a relay?  That would solve the issue.

60/55?.......The figures relate to the number of Watts, or units of electrical power the bulb will draw from the electrical system. 60 being high-beam 55, low.

Increasing the Wattage of the bulb will increase the amount of current flowing in the circuit (see Ohms law). If your friend now has x2 50W bulbs in the circuit instead of the original 1, there will be twice the current flowing in the circuit and through the fuse so the answer is yes..............its very likely to cause it to blow.

50W at 12v requires about 4.15A, 100W doubles it to about 8.3A.

+1 on the relay.