Worksman makes a tandem trike with single front wheel. If you look at it, you'll notice that the riders sit over the rear axle, with the rear wheels outside the riders, and then the single front wheel is quite a bit in front. I'm sure this would contribute to the stability quite a bit.
I'd say in your case, the challenge would be to find two old-but-identical bikes. I'd suggest to try to get some old 3-speeds which don't seem too uncommon or two pricey. Possibly also Schwinn Varsities, which are similarly common and cheap.
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