For 1 lamp application, cap unused blue lead, insulate for 600 Vrms

That's what it says on the bottom of the page that you linked to.

But I have two bulbs, not one.

Can't help but think I am running the two bulbs in series rather than in parallel. I'm thinking running them in parallel may be less stressful on ballast.

Tonight I am going to open the far end of the fixture and see if the wires are accessible to the lamp holders over there. If so I am going to rewire the fixture. But if the wires are inaccessible or too short, I'm not sure how big an issue this is.

"I found a ballast with 1 black, 1 white, 1 red, and 2 blue."

Wire color is not what tells you if the new ballasts s correct.

The ballast will be marked with what bulbs it supports and a wiring diagram showing how it should be connected.

The voltage would be VERY high to power 2 bulbs in series, and would likely fail quickly if one of them had a problem.

The open circuit striking voltages of ballasts are already very high.

I replaced some ballasts not long ago on fixtures I installed in 1958 in a church. They were used perhaps 8 hours per week with few on-off cycles. Amazing service life. But the new ballasts had wiring diagrams not even remotely resembling the old ones. I used the diagram on the new ballast. Go and do thou likewise.

I was able to rewire the fixture with no trouble. The light comes on almost instantly, while with the ballast hooked up using only one blue wire, it took several seconds to come fully on.

Thanks so much for your replies! Garden web is great!

Next time go T8.