Ace Double Books
by Andre Norton
There are 12 Ace Double Books (containing 15 titles) written by Andre Norton.
ACE then released another pair of titles "Star Hunter" & "Voodoo Planet" combined together but not considered an ACE Double book: although some sites have listed them as such (ACE released two prints).
If like me you have an interest in ACE Double books check out
the ACE Double Project section of this website.
https://www.andre-norton.com/special-sections/miscellaneous-stuff/157-ace-double-books-by-andre-norton#sigProId079c31f230