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First - a roll is a term generally used in kayaking to mean an Eskimo roll, so probably causing some confusion to others. Not something often done with SOT kayaks.
If you flip over a tandem SOT, you re-right the boat and should get on one at a time.
The first person to get on does a belly-butt-straddle move. Lunge up to get their belly over their seat (kind of like trying to exit the side of a pool when no ladder is available). Roll over so their butt is in the seat. Put their feet out to both sides to provide balance as they adjust themselves back into seats. The other person can help by going to the far side (at their seat) and holding on to the boat, helping to steady it and make it easier for the first person to get back on.
Once the first person is up, they can do some steadying by moving their weight around as the second person does the belly, butt, straddle procedure. Also, just by the first person putting their feet out to both sides (the straddle) and in the water will help provide balance (this is a common position for people in tippy SOT kayaks, like surf skis).
If balance is an issue, the two of you can try to get on at the same time, but should do it from opposite sides of the kayak.
There is an article that talks about this in California Kayaker Magazine - issue #7 starting on page 8. Can be read online for free at http://www.calpaddlermag.com/magazine.html.