Validity would not be the problem.
Assuming an episcopal consecration were done in the traditional rite with at least one of the two co-consecrators being a SSPX bishop from 1988, there would be no doubt about the validity of the bishop consecrated, even if someone had doubts about the principle consecrator.
Not all the details of the consecration have been announced yet.
Who else is going to be consecrated, and is it official that there will be co-consecrators?
+Williamson might have been fed false information, you never know.
You're right; even one valid bishop would be enough to confer orders. I would assume the SSPX would slip in a request to have +Fellay and maybe others be there, "just for the heck of it" which would keep the Trad faithful happy.