Hi Charles,
My usual setup is to join two tables along their long edges making a table about 48" wide by 60" deep. I am not sure if this arrangement will allow for those 5'x5' BB ply on your MFT/3 tables or not -- and don't forget to allow additional room for your fence... In any event, this is a useful setup that will require a 1900mm (75") rail to span the two tables.
Setting up multiple tables to accommodate 4'x8' material is another possibility. I've done this with 3 of the older 1080s to get a table about 110" long and mounted a 3000mm (118") guide rail on the guide rail brackets. That worked great for me, but took up too much room in my shop. I prefer to use a simple 4'x8' cutting table in my garage. I don't understand why some use multiple MFTs to cut 4'x8' material when they cannot swing a long rail from the guide rail brackets -- if you can't, then why not use a simple cutting table?
As for those cap screws, I was referring to the stops that are installed in the side profiles of the MFTs that are used to locate the guide rail brackets. On my older MFTs, the cap screws on these stops protrude from the side profiles and interfere with one's ability to connect tables along that side. You can use a spacer between the tables that allow for those stops, or simply replace the cap screws with some set screws as I described before.
Lastly, about using Qwas dogs. Steve made a very useful observation when he found just how accurately those holes are spaced. My thinking is that using dogs in the holes can work great for a single table, but can be problematic on multiple tables. Others may disagree. But, keep in mind that the MFT tops have a little slop within the outer frame, therefore precise alignment of the holes in the two (or more) tables may prove difficult to achieve and maintain. One problem I have with using dogs is table sag -- my tables have a little sag in the middle and therefore some of the holes are no longer perfectly vertical and the dogs do not all stand perfectly straight. This alone is somewhat minor, but the issue is compounded when I clamp the fence down and the overall result is that my fence is off more than an acceptable amount using this method. Making a brace to mount underneath to flatten the top(s) would help or even correct the problem for me, but I haven't done that yet. All said, for larger material I prefer jigs that sit on top of material to position your guide rails rather than do it MFT style.
Sorry for such a lengthy explanation.