help hooking up electrostatics 2 sets at a time

The Rowland amp, which is very stable at lower impedances, should have no problem handling two sets of Accoustats. Just run two sets of speaker cables from the amp to the two sets of speakers.
The Accoustat is a fairly civilized speaker, as it does not tend to go below 3.5 ohms. Some 'stats go down to 2 ohms, which can cause many amps to "fart in your general direction" and shut down on you! Running two pairs off that kind of amp will make them run away from home, never to return...)

Running two sets of speakers in parallel will halve the impedence. Therefore the amp will "see" 1.75 ohms. check with Rowland and see if the amp is stable into that low a load. If you are not using them in the same room, why not use a speaker selector switch?

Agree with above, do NOT connect these two pairs in parallel UNLESS you talk to Rowland first and get their firm OK. Swampwalker explains why.

If you much play 4 spkrs simultaneously you may have to choose an amp that provides for low impedance stability OR mod yr own for that purpose. Or, buy a second model5, of course!

You didn't say, but if you're using a pair of speakers together for L and for R (like people used to do with the original Quad ESLs), you might consider hooking each pair together in series. That would protect the amp and not screw up the frequency response of the speakers.

The amp would be driving a more "normal" load; and the only downside would be a little less volume out of each speaker before clipping. However, with 2 speakers per side being driven, you'd probably go deaf before you reached that point.

You might ask Jeff Rowland. The Model 5 was his first stereo amp, while his original 350 watt Model 7 monoblocks were used with Sound Lab A-1s. You can try the Model 5, but I don't know if it will be a good long term solution driving two pairs of ESLs. Go easy at first, and good luck!
Brian