So do I get it right that Wifibooth is not capable to Print from 2 Printers?
No, the good news is, that's not correct: Wifibooth will print
round-robin when printing to printers that are connected with AirPrint directly (ie. not blueprint). If you have two Selphys connected via AirPrint, let's call them A then B, and you connect them both in Wifibooth, then each time prints are made, the "other" printer will be chosen: A->B->A->B->....
There are two caveats to note:
1) as I wrote before, iOS is not capable of printing to >1 printer concurrently. That means that if you do a print of 1 photo, then immediate do a print of a second photo, the first will print on printer A, and the second will print on printer B, BUT:
printer B will not start until printer A has finished, even though printer B is idle. There's seemingly nothing I can do about this, it appears to be a limitation of iOS itself.
2) if you print multiple copies of the same photo (ie. user asks for 3 copies of their collage), these will all be sent to the same printer because this is considered one request. This is to make it faster to get the network data to the printer.
Hope that helps! In practice if you're using Selphys I did not find a second one to be of much help, because of the 1st caveat above. The only thing that improves is that instead of having ~18 prints before you run out of paper, you get ~36, but that's only if people make the same number of prints as each other (they don't)