Sorry but I can't even imagine how this would be possible much less a good idea! 5 gallon buckets are the smallest size container ever recommended for a single tomato alone. And even them the plant will be root bound long before the season ends.

Add sweet potato plants to that and the competition for nutrients and moisture would be quick death to both IMO. Not to mention what happens to the tomato roots when the sweet potatoes develop.

In a 35 gallon container - maybe with extras care. But in a 5 gallon bucket? No way.

Dave