if the bulb is really burned out, over voltage is the most likely cause.
no fuse wrong or faulty will cause this.
a short that causes another consumer to pull power through the bulbs, making the bulbs act as fuses is possible, but not very likely at all.

if the cause is over voltage, the regulator may not be at fault, but should be suspect.

the test-
with engine off, switch on lamps to make sure they're still working.
connect volt meter to bulbs (either side, but use the ground point at the bulbs for meter hook up), you should see something like 11.x volts with the bulbs on and the engine off.
start engine and rev to ~2,000 rpm.
load the electrical system by switching on things like the A/C, power windows, sound system, anything that requires a fair amount of juice.
I expect your charging system to operate within 13.2 - 13.8 volts. pushing 15 volts is not cool. in fact, at 15 volts I would say you have a problem.

if you do find higher than comfortable voltage start checking --
-voltage drop from alternator case to ground points at 1)battery negative post, 2)bare metal of engine and 3)any gang connection on the frame. half a volt (1/2) is the highest you wanna see in any case.
-as silly as it may sound, really make sure your battery connections are perfect (1/10 drop while cranking motor)
-if the alternator is mounted to the engine via vibration dampners, check the hell out of the alternator-to-engine ground strap. twice.
-check the smaller wires connected to the alternator, you should see a black plug on the rear/side of the alternator. with the engine off, remove this connector and make sure it's in good shape. a poor connection here will trick the regulator into an over charging state.

if you do find higher than normal voltage, the regulator may be the fault, but keep in mind that it's not that hard to trick a regulator into thinking it needs to go mad on the field winding in order to do its job.

there should be no more than 1/2 volt drop from the large lead on the back of the alternator (B+) to the center of the positive post on the battery with the engine running a good load on the system.

this all assumes the bulbs are realy burning out. the fact that they seem to die at the exact same time (left and right) is a little odd.but lets assume you know power is present at the bulbs when they're not working. (both the positive and negative side)

in 20 years of working on cars full time, over voltage is the only cause I have ever witnessed myself.

a few thoughts-
use 'Osram' bulbs, and never touch the glass to anything, including your fingers.
do not use high wattage bulbs (45-55 watts is normal)
make sure there is no after market alarm or fog lamps that may be wired in.