2 robots & 2 433mhz Rx/tx problem

Hi friends,

İ have 2 robots working with Arduino& DC motors and i control them with simple RF 433 MHz receiver and transmitter modules.First robot is lonely working very well.But when i turn on the transmitter unit of 2nd robot, both of the RC system does not work.after then i turn off the transmitter of 2nd robot and the RC system of First robot starts to work again.

Where May be the problem? The software or the Hardware?

in the software of both , for example direction messages belongs to transmitter units of robots are different.

For example;

When there is no button pressed in transmitter units, tx of 1st robot sends "1" , tx of 2nd robot sends "A" to the receivers on the robots they are belong to.And in every message combination of diectional movements the messages are different as well. Does it make a problem? İ really dont have an idea being as maker so please help.