I have a series of 3x9 raised beds and grow tomatoes on one foot centers (starting 6" from the edge to fit a total of 9 plants) as single stems as a row down the back of the beds, then plant 3 or 4 hills of sweet potatoes at the front of the bed, usually with 2-3 slips per hill. For the peppers, I plant 6 pepper plants down the center of the 3x9 bed and then plant 2 hills on each side of them for a total of 4 hills of sweets. With the peppers, I grew the more bush type vine variety called Vardman and under the tomatoes I grow a beautiful blue sweet potato that vines like crazy. I think as long as you don't let your tomatoes be such bushy beasts they totally shade the sweet tators you should get some potatoes, but I definitely notice that the more shaded the vines for a given hill is the less productive.
I believe one of the reasons the approach works well is that the sweet potatoes shade the soil and keep it from drying out and overheating. When the soil is hot, the tomato plants are less able to take up the moisture and nutrients in the soil, so it really seems to help keep them healthier through the summer heat. And when they really get going the sweet potato vines do a great job of suppressing the weeds as well.
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