Here are my GE F14T12/CW and GE F14T12/D running in series on my Advance L-140F-TP 30/40 watt preheat ballast.

This setup works really well. Total arc voltage is 87V, more than the minimum the ballast was designed to handle. Lamp current is 0.39A, which is nearly perfect, and line current is 0.72A, which is below the ballast's rated maximum, so the ballast shouldn't run hotter than normal, either. All-in-all, I say the Advance L-140F-TP would make a fine ballast to run two F14T12 lamps in series.

I plan on conducting a bunch of experiments running various combinations of lamps in series on this ballast, measuring lamp and line current, and uploading photos of each. If anyone has any specific combo they would like to see, feel free to suggest it to me.