Personally, I can think of three right off the bat:
1. Stereopony - The two groups know each other to begin with, and have a similar musical style, so it would be pretty much a seamless fit. Both bands can rock and do ballads, so I don't think there'll be much of a problem picking a musical genre.
2. Perfume - Just like Stereopony, these two bands know each other well too, however the severe difference in their genres might be a hurdle...of course, I wouldn't mind an electronic remix of some of SCANDAL's songs, or on the flipside, a non-electronic rock version of Perfume's songs.
3. JUDY AND MARY - Daydream combo anyone? Of course, this is strictly an imaginary suggestion, considering the age difference and JUDY AND MARY having disbanded a long time ago.
4. Chatmonchy - It will be hard, considering the difference in voice style between Chatmonchy's lead and Haruna, but well, sometimes the most unthinkable of combos combine for a nice mix