Would it make a difference if I put, first the variable power supply and second a not variable power supply in series than the other way around (first not variable PSU and then variable PSU)? What would be preferred of not?
Depends on the power supply, Suppose the supplies were 1000 V 0.1A. My crystal ball is broken.
Proably not. Suppose they are 12 v 10A; Maybe. Suppose they are ATX supplies - NO!
There is a specification on how far you can float instruments from ground. $1000V is probably too much. ATX power supply have the negative 12 connected to chassis ground. So no.