Valbazen is given orally, technically by pound at a rate of 0.08ml per pound of weight, so a 5 pound chicken would get a dose of .45ml once then repeated in 10 days.
As far as I know Valbazen settles out of water, so I don't believe it would be that effective if you just put it in the flock's water.
To answer your question though - Yes. Corid (Amprolium) and Valbazen can be given simultaneously. I find no contraindications in the information I have.
@casportpony or
@dawg53 may have better ideas about administration of medications.
I do know that you can get a "formula" of Fenbendazole called Safeguard Aquasol that is specifically made to go in water, but it's very expensive.
I'm very sorry to hear about the Marek's. I know you have struggled with the disease. It's good that you have found a way to hopefully keep them healthier. I'm not questioning your method at all, but do you feel that the disease weakens birds that it allows Coccidiosis and worm overloads?