Only been to the MEN once but it looked as big inside as the O2 and the published capacity numbers are almost identical IIRC.
The situation if you had two arena venues in operation in MCR would be a bit like Brum where the NEC (or whatever they are calling it today) is on the outskirts and the Birmingham Arena (formerly NIA, Barclaycard Arena etc) is in the city centre.
The problem is having enough artists who can fill arenas visit on a regular basis. Designing in real flexibility on sizing the venue seating so that it doesn't look empty for "smaller" gigs would be interesting.