Playing with B teams is kind of... well challenging, but annoyingly stupid.
In one save I started unemployed, got offered to manage Espanyol B. Everything went really well in the first season, we were favourites to drop, but I manage to finish 5th.
The manager of the 1st team was a douchebag and wouldn't let me sign average players to fill in the squad as i only have around 15 players and ALL the good players I had were taken to play into the 1st team, which was a shame.
You are basically managing a team in which you don't really have much control of anything, pretty much just tactics.
2nd season I got sacked halfway through the season while at 17th place with a crappy 15 squad Espanyol B team. Good luck to the next manager in line (He got sacked pretty quickly...)
Anyways, I got sacked, then I managed CD Ourense, which was better.