////////// bar.c
#include <stdio.h>

void bar ()
{
    printf("bar\n");
}

/////////// foo.c

#include <stdio.h>
#include <mach-o/dyld.h>
#include <mach-o/loader.h>

void onLoad (const struct mach_header* h, intptr_t slide)
{
    printf("loading image\n");
}

void onUnload (const struct mach_header* h, intptr_t slide)
{
    printf("unloading image\n");
}

void foo ()
{
    printf("foo\n");

    _dyld_register_func_for_add_image(&onLoad);
    _dyld_register_func_for_remove_image(&onUnload);
}

//////// main.c

#include <stdio.h>
#include <dlfcn.h>

void loadBar ()
{
    void* handle = dlopen("bar.dylib", RTLD_LAZY);
    void (*bar)() = dlsym(handle, "bar");
    bar();
    int r = dlclose(handle);
}

void loadFoo ()
{
    void* handle = dlopen("foo.dylib", RTLD_LAZY);
    void (*foo)() = dlsym(handle, "foo");
    foo();
    int r = dlclose(handle);
}

int main (int argc, const char *argv[])
{
    loadFoo();
    loadBar();
    return 0;
}

$ gcc -o foo.dylib foo.c -dynamiclib
$ gcc -o bar.dylib bar.c -dynamiclib
$ gcc -o main main.c