Is it possible to submit a PhD thesis with two authors?
It is definitely possible in the sense of "It has been done." However, the only example I know of is this thesis from linguistics: http://dare.uva.nl/search?arno.record.id=123669 (Groenendijk &amp; Stokhof 1984)

Whether it is possible in the sense of "You could do it." will depend on your university's regulations concerning these matters, as well as your advisors and committee's preferences. They are the people you should talk to about this.