Just another thought, the first thing I would assess is the skill of the two folks who were paddling the tandem. If that are just tourists first time in a boat you will be back to basics. But the couples I know who paddle tandems by preference have also usually worked on rescues per the technique described by Peter-CA. In that case you may only need to provide minimal assistance.
You absolutely should try to do what Peter-CA described, but be one of the swimmers yourself in a tandem unassisted rescue. Works in a tandem kayak or in a canoe. I was floored how easy it was the first time I did this with Jim. He outweighed me by 60 pounds at the time. I got in first with him holding the canoe. Then I sculled, wedged and leaning out from the bow of the canoe to balance him getting in. The coach had to tell me to stop. It had been so easy that I never felt his weight coming into the canoe.
If you can do this yourself you will be in a lot better position to help others. You will understand what is going on with the other boat.
As to lifting the boat, what you should be doing especially at the size we are is getting as much water as possible out of the other boat first BY ANY MEANS POSSIBLE. To break the suction, that may mean flipping it upright as a starting position. (ask the swimmers to help.) Then roll it on its side as you start bringing it over your boat. And edge your own boat to help as you start moving the other boat out of the water., don’t just lift it. As long as you have hold of the other boat, you can edge the hell out of your own boat and you will not go over.
You probably need to pass the entire length of the tandem over your own boat to get both cockpits emptied, same as a canoe. But it is just doing the same thing on both sides. Once you have the boat solidly moving out of the water it’s not big deal.
One of my major departure points from the BCU was when they insisted on co-mingling canoes and kayaks to move ahead in the awards. It was a dumb idea but worse was they enforced it inconsistently and as a result unfairly. But I did walk away from some of the rescue work with a conviction that all kayakers should also know how to do rescues with a canoe. It is excellent practice and can translate to other situations like tandem sea kayaks. BTW, I am not including the fully open rec tandems, I have been with others trying to rescue one of them. Happily is was a practice session because frankly nothing worked.