This baffles me. 185-operated TPE routes are by and large overcrowded, sometimes seriously and at all times of day, because of the grossly inefficient layout of the Class 185 that gives it a capacity more like a 2-car unit. Northern double-unit routes are not necessarily. I'd rather have a pair of Pacers with spare seats than a single 185 without.
I imagine most passengers don't see it that way and want a pair of modern 185s, rather than a pair of older 156s. Average Joe has no idea how many of each type of train were built so wouldn't be aware that the only way of getting doubled up 185s on North West routes would involve a downgrading of cross-Pennine services.