Give me a pairing, threesome or moresome, and I'll tell you my headcanon for...
1. What they most commonly do during sex (if applicable)
2. Who has the prettier paintjob/sexiest style
3. What they argue about most often
4. Who'd cope best if the other one died
5. The happiest plausible happily-ever-after I can think of for them
6. The most tragic possible ending for them