In which case why do people bang on about a 3-car 323 being better than a 4-car 319 when neither are ever doubled up?
And nor, I suspect, will 331s.
The plan for doubling up 331s was for West Yorkshire services in the peaks, with them being run as single units off peak. It seems that Northern have come to the conclusion that it would be overkill. By switching 4 coach 331s to Yorkshire they would free up double the number of 3 coach 331s for the North West. If that is the new plan then it is not clear how they will be used.
There is demand for 6 coach EMUs through Bolton but the service would need to be carefully chosen. It would be cheap to extend the platforms on the Bolton line to 6 coach length but Salford Central would be very expensive. A single 319 or 323 would be adequate for them. The simplest solution would be to double up the Northern Connect service to Blackpool. If the 4 coach 331s go to the North West then the other Piccadilly-Bolton service would be the next best option (using double 323s). With short platforms at Salford Central and terminating at the less desirable station, I can't see any Bolton-Victoria services being run by 6 coach sets.
6 coach EMUs are probably inevitable for the Piccadilly-Bolton services within the next 10 years because the Castlefield capacity shortage will limit services to 2tph (excluding pick up / set down TPE services). Its easy to forget that a 323 has more seats than a double pacer and only 10% fewer than a 319. Both could be used too e.g. 319s for Victoria and 323s for Piccadilly services.
Edited - Salford Central not Crescent