That's not a very clear example. You don't need to be using multiple versions of the same dependency to contend on access to stderr/out, just having a println in your code along with logging code will have the same effect.

I haven't ever observed a problem of concurrent access to stdout/err though, I expect because the methods for accessing stdout/err lock them for the duration of their printing. If you Google for "Rust print console slow", you'll probably find advice to explicitly lock it, to avoid individual printlns from each acquiring the lock.

"The same heap" isn't a coherent concept here. Your malloc-implementing memory allocator has some global state, and that state has some pointers to some addresses it got from mmap and some metadata about how long those spans of memory are, which parts are unused, and how long the values it has returned from malloc previously are. If you managed to use two of these, they would each contain data referring to different non-overlapping sets of memory mappings. If you accidentally used a pointer from one with the other, you would go instantly to C UB land:

  The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.