how to call threads from other c program

Question

I have a program which is creating two threads but all of this is done in a single c program here is the code

#include<pthread.h>
#include<stdio.h>
#include<stdlib.h>

void * thread1()
{
int i=0;
    while(1)
    {

        printf("%d Hello!!\n", i);
        i++;
    }
}

void * thread2()
{
int j;
    while(1)
    {
        printf("%d How are you?\n", j);
        j++;
    }
}

int main()
{
    pthread_t tid1,tid2;
    pthread_create(&tid1, NULL, thread1, NULL);
    pthread_create(&tid2, NULL, thread2, NULL);
    pthread_join(tid1,NULL);
    pthread_join(tid2,NULL);
    return 0;
}

now if

void * thread1()
{
int i=0;
    while(1)
    {

        printf("%d Hello!!\n", i);
        i++;
    }
}

is defined in another program for example mythread1.c

and

void * thread2()
{
int j;
    while(1)
    {
        printf("%d How are you?\n", j);
        j++;
    }
}

is defined in another c program for example mythread2.c then how can i call these in my test.c program

test.c

int main()
{
    pthread_t tid1,tid2;
    pthread_create(&tid1, NULL, thread1, NULL);
    pthread_create(&tid2, NULL, thread2, NULL);
    pthread_join(tid1,NULL);
    pthread_join(tid2,NULL);
    return 0;
}

Please guide me is there any way to do this using Linux system call!