Going to take a SWAG here, might be helpful, might not.
First a few questions-
fiberglass tilting hood?
do park/marker lights work?
how bright are headlights?
My first guess is ground wire failure. On some models IH used one ground wire for headlights & a seperate one for signals/markers.
IIRC, there will be a terninal strip, or a plug, where the harness passes from the hood to the frame, handy spot to test for power.When checking for power at the lights, connect the ground for your tester directly at the battery. Dim headlights would be a sure sign of bad ground, whether common to signals or seperate.
If indeed there is a ground failure, the most common spot is where the harness passes from the hood to the frame rail at hood hinge/ radiator/ frame junction. That harness has to twist a bit every time the hood is opened/ closed. HTH
p.s. Or even the simple failure, by coincidence, both bulbs failed at the same time. Have seen that happen. Spent some time working school bus shop back in mid 80's. About 50 busses pounding along gravel roads, bulb failure quite common.