Each DevTest server component runs in its own memory space (JVM) so there is not really a concept of two DevTest components sharing the 'same' memory. Each consumes its memory from the RAM allocated to the server on which the service is executing.
One approach to adjust memory is provided using directives (i.e., -Xms, -Xmx) in the VMOPTIONS file of a given service. These adjustments only affect that service. Reducing memory too far could result in unexpected crashes (out of memory exceptions) or poor performance.
Are you seeing an unexpected memory utilization on your machine(s) based on the DevTest memory requirements? Version 10 requirements can be found here: System Requirements - DevTest Solutions - 10.0 - CA Technologies Documentation Your specific version may be slightly different.