Single or double thesis for double major?
Write one thesis, so you only have to write one introduction and discussion section, and only have to go through the thesis formatting nightmare once. If you prefer two small projects, you can make two projects two separate chapters for your thesis and just try and figure out a way to relate them in your introduction and discussion. 

Make the title of your thesis sound applicable to the field of study you want to get your masters in (no one will read your undergrad thesis on the admissions committee and I doubt anyone will care whether you wrote two theses or one). 

You say you have a low GPA. Perhaps two smaller projects (still for one thesis) will allow you to collaborate with two different professors. Why is this advantageous? Because that is potentially two very strong letters of recommendation. Many undergrads have one good letter of recommendation and the rest are just professors that taught a course they took. If you have two very strong letters, that is quite impressive, and will do more to get you into graduate school than anything related to your thesis, short of publishing it in a peer reviewed journal.