Hmmm, just bc the vet said it was "okay" doesn't mean it will actually work better.
Some drugs are potentiators of other drugs. For example, if you have a headache - it's great to take caffeine w/ aspirin bc caffiene potentiates the effect of the aspirin = headache gone quicker (plus caffeine works nicely w/ vasoconstriction too
).
The point is, I don't know if Temaril and Benadryl actually potentiate each other, "act" as a stronger anti-H if taken together, or do absolutely nothing beneficial therapeutically if taken together - and if your vet can't actually answer that question, that's when you have to call a pharmacist or 1800 PETMEDS (or whatever) and see how taking those 2 drugs together would work (or not work).