The somonka is a Japanese form. In fact, it's basically two tankas written as two love letters to each other (one tanka per love letter). This form usually demands two authors, but it is possible to have a poet take on two personas.
The tanka is a thirty-one-syllable poem, traditionally written in a single unbroken line. A form of waka, Japanese song or verse, tanka translates as "short song," and is better known in its five-line, 5/7/5/7/7 syllable count form.
You can collab if you want to! Get inking writers and have fun.