If you do a combined multi/CPL you save money straight away, plus you then have 10 hours multi time before you start the IR. Helps get off to a flying start. A lot of schools couldn't be bothered getting a complex single for the few students doing a single engined CPL, so they can be reasonably competitive on the twin prices. For instance, I did my Multi CPL at Leeds FLying School at LBA. Instead of 15 hours single and 8 hours multi, I only did 11 on the single and got a competitive rate for multi-time for the remaining hours. I now have 16 hours multi time. The CPL test doesn't seem to be any harder for although you do single engine procedures (asymetric approach etc) you miss out forced landings. So swings and roundabouts.Most twins use an airswitch for recording times (i.e. what you are cahrged) but you log taxi time also...which is important at a busy field like Leeeds where they seem to dislike having GA aircraft on the runway or on approach. (Only kidding LBA ATCOs)
My advice would be to talk to a GOOD school and get their price for a multi CPL vs a single engined one.