These 2 sites are 2 different tenants having different databases.
Is there a way to interchange data between the 2 tenants/sites through REST API?
If cannot be done through REST API is there any function or mechasim in order to be able to interchange data between 2 ERPNext sites in real time?
Yes, its possible. You write a python script that connects to both instances and exchange data as you need. You can follow the example code in frappeclient github. If you need specific help, send me a pm.