as a general rule if 1 blows the other isn't far behind (unless it's caused by hitting that particular globe or something similar that isn't just age/wear and tear) so it's good practice to replace both at once, keep the one that is still working in a packet in the glovebox for a spare.
now some simple maths and i'll ignore low beam, P(ower)=V(olts) X A(mps) so P / V = A and i'll just use 12 volts even though it's really closer 13.8-14.4V
55/60W globes, 60W(H4) x 2 + 55(H3) x 2 = 230W, 230/12=19.166666A, we'll round it upto 20A for good measure and this is what the standard globes will draw
90/100W globes, 100W(H4) x 2 + 55(H3) x 2 = 310W, 310/12=25.83333A, 25A isn't enough to blow the fuse but will create excessive heat on the blades.
90/100W globes and 100W H3's if someone puts 100W globes in all 4 high beam sockets, 400/12=33.3333A, this is where people really come unstuck and they will put a higher amp fuse in and the wiring & fuse holder just isn't up to handling close to 15A more than what it was designed to do.
xxx/130W H4 & 100W H3, 460 / 12 = 38.3333A and the list goes on to whatever combination of globes is used.
Apart from the wiring not being upto scratch with age (the older the cable the more corrosion will be present playing havoc with connections and splices) and it simply wasn't designed to be carrying these loads, to high a wattage globe (especially in newer cars with plastic headlights) can damage the housings and lenses.
