I think the idea behind this is to keep them from having to call down the line for parties of 2 to fill in ride vehicles around larger parties.
On TSMM, if you went as a party of 2 adults and 1 child in this line I believe you will all end up in one row. They really mean it about SMALL parties of three for this as the ride is pretty squishy with a kid and two adults. But here's the real kicker- the kid (or whoever is sandwiched in the middle) won't have a gun and won't be able to "play" the ride. So I would only recommend doing this with a very small child who won't be able to operate or play the ride anyway.
This is less of an issue for Monsters as there isn't an "interactive" aspect to the ride but again your whole party will probably be expected to sit in one row. This is doable but again, how squishy depends on the size of everyone in the party.