The second compiler can be subverted, as long as it's not subverted the same way. In particular, it could be one you wrote yourself. It doesn't need to support

The second compiler can be subverted, as long as it's not subverted the same way. In particular, it could be one you wrote yourself. It doesn't need to support everything, just enough to compile the compiler under test. It could also be a compiler from a different group/organization. You could use many compilers and test with all them.