You can graft them but so far no one has cross pollinated them . Try a paint brush (small artist type )to tranfer pollin.

how do you graft them

They are called topotates and can be bought already grafted . They bear lousy tomatoes and lousey potatoes on the same plant . Or you can make your own by growing a potato and a tomatoe next to each other, when they are long enough to stretch across each other and cross in the middle of each other . Then put a small notch in each so they fit together and tape them together ; let them fully recover . Once plants are strong again cut off potatoe top let them grow until tomatoe top shows significant growth ten cut off tomatoe bottom . This will leave you with a tomatoe top potato roots . good luck !