Hi Mark.......years ago you sepped a job for me in LA. It was Mario Broos or something; sim process on black on a 14 color press. This was the only time I was ever given two underbase white screens. A 355 ultra high detail and a lower detail maybe 230 printed second wet on wet. Then flashed with about ten spots. 65 LPI. The intention was to create the brightest values of white where needed (lower mesh) while insuring the subtler values of white resulting in superior shades and tints of subsequent colors (higher mesh).
The graphic (from Pixar?)had an insane amount of colors and it was all up to the underbase to do the work. We were pretty amazed at the outcome....certainly the nicest piece I had done up to that time. First kudos to you for coming up with that solution; second I'm curious as to how often thats done.
Oh and a sign of the times I believe the seps cost $1,100!
Best tp