1. A sexual act in which two fully or partially nude men face each other with their penises lined up lengthwise, at which point one of the men puts his hand below or to the side of both penises and begins stroking. The name is derived from the resemblance of two penises in one hand to two hot dogs in one bun, although if you're doing it right it will look more like an open-faced sandwich.
2. Same as above, but the hand belongs to a woman.
"Why give each other consecutive handies, when we could save time by enjoying two franks in a bun?"