Yes, I have two 3880's and four others, but the two 3880's are the main ones I use the most for prints.
I switch papers a lot. I have seven 17" rolls by me now and a stack of Red River, Inkpress in 17"x25" boxes (I prefer to cut the 17" rolls to 25.5" inches as that fits the 24x36mm format perfectly and no cropping.). Problem is some take MK and some take PK, but sometimes I don' see a visible difference in the two blacks either. Depending on the printer, software, driver, ink type (OEM, Cone Pigment, K7, or Dye), and stored profiles, mistakes happen and if it switches, I have to go back yet again (i.e. "I wanted PK, but an accidental switch occurred, so I have to go back.") so the "accident" was a wasteful two-flush switch.
That's why the question if one black ink can serve both MK & PK providing I can do a GO coating for some black ink that could be used in both tanks if it is a gloss or bronzing issue, as well as do a profile to address any dMax issues doing so.
Imho, Epson really bites for that wasteful PK>MK switch as well as ink drying out in the other line at times, but they may like ink sales from doing so more too. Should have had another nozzle set for it and maybe a small sputter of the other while printing just to keep the flow going.