Batteries cause a disproportionately large number of problems for such an apparently simple device. A little rudimentary maintenance can prevent most problems, although it should be realised that batteries have a finite lifespan, which will be shortened if they are not maintained correctly.
DMUs are 24 volt. All vehicles I've ever seen have two sets of cells, one on each side, connected in series to provide this, and both are required to provide any power. Both sets are needed to start either engine!
A set of cells with one dud one will never start an engine reliably, however much the others are charged.
Batteries with two negative terminals and two positive terminals have them connected internally, and do not require all four to be connected to work. However, connecting all four terminals provides a degree of redundancy and reduces problems with bad connections.
Starting engines with 'boost' chargers will only work if the batteries are in fair condition anyway. A charger won't start anything unassisted. Jump starting from a couple of large lorry batteries (with suitably heavy jump leads) does work but car batteries aren't usually up to it.
I have yet to hear of any success with the 'Battery aid' tablets sold by various places. At the outset, DMU batteries are much larger than the car batteries these tablets are intended for, so a correspondingly large number would be required. As to what they are supposed to do - I don't know. Suffice it to say that attempts to revive a couple of dead car batteries with them proved futile and I've not tried again.
Batteries cause far more problems than they should. Ideally all power cars should be alternator-fitted and all others charged regularly. As with so many other things, the proof of the pudding is in the eating - do your engines start or not? A reasonable life for an regularly-used power car is 5 years and for a trailer 10 years; start saving up the pennies as replacements are expensive!