The correct answer depends on several variables: terrain, fitness of the singles and tandem, and skill levels of all play a role.
If both tandems are fit enough that they have more horsepower than the singles, I'd just rotate the two tandems on the front on flat or mildly rolling terrain.
If everyone is more equal in power it gets more complicated. I'd likely keep the two tandems together, since their differing momentum should be more manageable that way. Then I'd just rotate the paceline in the normal manner. If everyone is a skilled paceline rider, it should be steady enough that the differing characteristics of the tandems don't create havoc.
In rolling terrain you can start to strategize on when to have the tandems pull: Generally you want them coming off the flat into a roller or descending off the roller. The rest of the time the singles will pull, and will likely need to adjust to stay in contact with the tandems.
We were riding with another tandem and a halve dozen singles on mildly rolling terrain last weekend and it went better than I expected, though we did drop one of the singles when the tandems went to the front on a fast descending section of road.