Although a potato and a tomato cannot be cross-bred, it is possible to graft together a tomato plant shoot to the rootstock of a potato plant. The grafted plant will produce both tomatoes from the tomato shoot and potatoes from the rootstock. Grafting together the two plants can save you space in the garden and make a tomato plant grow that is not well suited for the soil in your garden. Be aware that the grafting together of the two plants can effect the taste of the fruit.
Cut the stem of the potato plant so that it is split into a "V" shape. Cut the stem so that about 1 inch is exposed above the ground.
Cut the roots from the tomato plant, making a cut on the stem that is straight across using a sharp knife. The cutting should be about 6 to 8 inches long.
Cut the tomato stem so that it looks like a wedge shape by cutting material off both sides of the bottom of the stem. Make the cut so that it fits into the "V" shape cut into the potato plant stock.
Slide the two cut pieces together.
Wrap the graft with grafting tape so that the cut holds.
Remove the grafting tape once the plants begin to show new growth.