Guile doesn't. You'd use one interpreter and isolate whatever you need to isolate in it's own module and if you need things in parallel, you can just use pthrea

Guile doesn't. You'd use one interpreter and isolate whatever you need to isolate in it's own module and if you need things in parallel, you can just use pthreads (guile has proper threading support and no GIL). If you don't cross the C barrier again from your scheme code you can even use something like guile-fibers for Concurrent ML-like parallelism.