Welp, after my long post on Feb 8th, I solved my knee twinge issue by switching the entire crankset, including shorter cranks, to the other bike.
The steel bike with 172.5 cranks and round 50/39 chainrings didn't give me any problems. Nor did the 170 cranks with 52/42 Biopace on the carbon fiber bike.
But swapping the Biopace chainrings to the bike with 172.5 caused minor knee problems that persisted for a couple of weeks.
So this week I switched the entire crankset -- 170 cranks and Biopace rings -- to the steel bike. Knee problem gone.
I'm surprised such a minute difference in length made so much difference. But the eccentric, oblong Biopace rings felt surge-y, pulse-y and weird with longer cranks. Paired back with the original shorter cranks they feel right again. The shorter stroke feels more efficient to me.
Now I'm rethinking my cavalier attitude toward crank lengths. I might just switch all my bikes to 170 now. At 62 I've managed to avoid serious knee problems despite many accidents and a family history of deteriorating knees. I'd like to keep 'em healthy as long as possible.