Now, Darius fetches the code from the same source as you will do, that is true. But does he use the Local Lab branch? I doubt it, I presume that he offers the gtk3 branch.
If you follow the instructions that I gave you this morning, your RT will be the Local Lab branch, and your RT will be placed in folder ~/rt.
Darius' RT most probably will be placed in one of the bin folders in your computer, meaning that they both (i.e. yours and his) can coexist in a friendly manner -- you simply decide which one you would like to start from time to time.
I cannot see that this will be a drawback, but if you have several RTs on your computer, they will all share the same config file (located in your home folder).
If anything should go wrong, just delete them and reinstall the version you would like to keep
I repeat: if I can do it, so can you!