What spool is driven while starting?
A claimed benefit of the RB211 and other 3-spool turbofan engines is that they are "easier to start, since only one spool needs to be turned." (Ref. Handling the Big Jets, Davies).
I presume the spool that is turned, through the accessory gear case, is the N3 spool. So how many spools are normally turned on a 2 spool engine?