Aside from the Stratolaunch, how many other aircraft have split bodies?
Depends on how you define it but there have been a variety over the years: 


The P-38 Lighting has 3 bodies there was also a later experimental version the XP-49 that only saw a single airframe. 
The F-82 was a a twin fuselage version of the P-51
The boomerang which was also designed by Burt Rutan (the designer of the stratolaunch)
A one off twin fuselage piper cub was built but never saw production scale. 
As @DanPichelman points out in the comments, the Yak-110



  ...although launching spacecraft from them commercially is a new thing


This is not entirely true, piggy backing rocket planes on larger aircraft has been widely done since the 50's