I want to create two different applications. In first application user should be able to access the camera of other devices and in the second application, user type in one device via keyboard and it should be displayed in other device.
For this purpose, it is mandatory to access other mobile's application via bluetooth or Wi-fi?
How does it be possible?
It is not mandatory. You can also implement this as a server client solution, where the data transfer between the clients takes through the server. If you didn't try it yet, that will be a tedious task for the particular solution. Both side should keep services to monitor and transfer data.
But if you intent a real time solution, I'll prefer Bluetooth for faster response.