Same as above, I haven't cloth nappied twins but my two were both in nappies at the same time for just a month or two when my youngest was born and it really was fine.
I personally wouldn't decide how often you want to wash and then buy enough nappies because your machine will still only hold the same number of nappies. While it's true to some extent that you can buy more nappies for two kids, your limit will be how many nappies you can fit in one wash load. For a standard size washing machine (I think that's about 6kg here), you're probably looking at 18 nappies in the wash at once max, although it depends on the type of nappy as some take up more room.
You might find that where a parent of one could wash every third day, you will need to wash every 1.5-2 days (and I would count on putting a wash on every day at the start, until they poo less frequently), but it really isn't difficult.
