Ah, the good old CMOS camera lines.

If you have access to a tulamp lead/lag ballast, take a picture of the lamps running since you should see a different flash pattern for each lamp.

This ballast us for two lamps, though I'm not sure if it's lead/lag. From this pic, if i remove the lamp on the right, the one on the left will still light, but will be very dim. If i remove the left lamp though the right one won't light at all though.

Since your ballast is RS it should be a series model which is why both lamps have the same flash pattern and why the lamps won't light fully if you take the other out. There is a way to run a single lamp on these series RS ballasts though but it's not the ideal way...you have to connect it using the red and blue wires and cap the yellows separately.

I believe only preheat and slimline ballasts were ever made in lead/lag configuration. Although I think most newer slimline ballasts run the lamps in series instead.

With the lead/lag design, does one lamp stay lit when the other goes out?

Yeah if the lag lamp fails (blue ) side then the lead lamp stays about 50% brightness on the slimline ballasts usually lag lamp failing overheats the ballast as the lamp rectifys and the DC current backfeeds through the ballast
if the lead lamp fails it looses vaccum usually and no light at all

How come RS ballasts have the lamps operated in series while preheat ballasts are in parallel? Have they ever made one starter for a two lamp fixture?

Yeah its possible with an F40 preheat ballast 2 20W lamps in
series