Metra Electric runs double deckers in married pairs, but:
- Highliner II is taller than MLV
There is no huge double tap transformer for 25Hz plus 12.5kV vs 25kV, Metra Electric is 1.5kV
While Metro-North has a different constraint on its car geometry, it is worth pointing out that they did opt against a double tap 25Hz transformer for the New Haven Line EMUs (relevant for the Penn Station Access) with the major objection as far as I know being the weight of the iron transformer core.
While in theory NJT could try to create time separation with freight for the lines they own, they do not have that authority on the NEC, so non-compliant equipment is not an option.
Yes, the ALP45s are here and running, but they are overweight, need more maintenance and more frequent refueling (and burn more of it per mile) and cost much much more than conventional equipment. The fact that nobody else has ordered any is quite indicative. Indeed, even Montreal is planning to get rid of theirs eventually. The fact that the ALP45s exist does not make them worth it.
Back to the MLV EMUs, I never tried to make the point that married pairs cannot be done, just that they are not worth all the trouble. Yes, you can stuff everything in a married pair, but do you get the extra seating? And if you get some, is the overall cost worth it? So 3 manufacturers have figured out a way to stuff everything in there. That does not mean that the cost is worth it. NJT might get them anyway, but again that does not make them worth it.