Q400 Captain reports inability to receive on communication radio 1 when the other pilot is transmitting on communication radio 2. This problem seems to be fleet wide and has gotten worse over time.

Narrative

During initial descent; the PM left ATC frequency to contact the station. While the PM was making several transmissions on communication radio 2; ATC attempted to issue a new clearance to us over communication radio 1 but I was unable to hear any of it (only garbled static) due to bleed over interference from communication radio 2. I was not monitoring communication radio 2 on my side; yet it still disrupted communications with ATC. I recognized that it might have been a clearance meant for us; so I told ATC that their transmission was completely blocked and asked for it again (hoping that it wouldn't be blocked all over again since I noticed that the PM wasn't done communicating). The clearance was to descend to 10;000 FT.The frequency interference is getting worse on the Q400. I've encountered multiple events where this type of disruption to communication with ATC becomes operationally problematic. Another symptom I've noticed is that when someone else (another aircraft; ops; Maintenance; Dispatch) is transmitting; bleed over (static) is noticed on communication radio 1 with communication raio 2 deselected. Either demand a complete fix from the manufacturer or initiate a refit with new/different radios.