Can a Lionel ZW share common with MTH 4000?

Both are isolation transformers, so no reason why you cannot connect common.

That said, anytime you use more than one transformer and connect them via common- be sure to check phasing. In this case, since they are slightly different transformers, the test result might show some voltage, but the difference is still obvious (in-phase vs out of phase). Simply adjust an output on each transformer to 18V, then measure using a meter between those 2 outputs. If the error is low- only a couple of volts- that's in-phase. Out out phase, the 2 voltages add up to something like 30+ V which is why we check phasing in the first place- to prevent higher than normal voltage situations on the layout.

Yes! I've been doing this for 22 years. Just follow @Vernon Barry advice on phasing. To change phasing on the ZW, reverse its plug in the wall socket.