I use 2 PCs in a network to fly with the flight simulator.   
On one PC (called Master) I installed P3D my airplanes from PMDG, FSlabs and FS2Crew.
On the other PC (called Client) runs IVAP or VPilot, Active Sky, Navigraph Charts, the ACARS of my virtual airline and Webeye
The communication between the two PCs is done with Simconnect and FSUIPC registered with WideFS. With FSUIPC/WideFS it is possible to transfer the PTT (Push to Talk Button) command from the master to the client. How to set it up is described in detail in the FSUIPC documentation.  
IVAP or VPilot running on the Client require a microphone for radio communication. I have an on ear headset with microphone connected to the client. I can hear the radio traffic over the headphones.
Fs2crew must be installed on the Master. Fs2Crew is loaded with the other instruments of the airplane and interacts with the airplane like an instrument. Therefore the microphone with which the voice commands are given to FS2Crew must also be connected to the master.
The answers I get from the copilot (Fs2Crew) I hear through speakers connected to the Master. I use other loudspeakers than those I use for P3D by defining the soundcard as headset in the FS2Crew settings.
Now all I need is a microphone connected to the Master for voice input to Fs2Crew.
I found 3 possibilities for this:
1.)    A second microphone connected to the master, for example a desktop microphone suitable for speech recognition. Such microphones are very expensive, so I've never tried them before.
2.)    Y-Splitter cable. For this you need a headset with 2 jack plugs. One is for the microphone and the other for the headphones. With the splitter the microphone signal can be divided. You can use it to send the microphone signal to the sound card of the master and the client.
I've done this for several years. However, the sound quality is not very good. By splitting, only one channel is transmitted to the sound card at a time. In addition, the two sound cards are electrically connected by the splitter cable. Depending on the two sound cards, very strong humming tones can be generated, which further deteriorate the speech quality.
After buying a new PC with a new sound card, the transmission of the speech signal was so bad that my copilot (FS2Crew) hardly understood anything anymore and the ATC at IVAO or VATSIM also had considerable difficulties to understand me.
3.)    Software solution
Voicemeeter by VB-Audio Software (Copyright V.Burel ©1998-2018)
https://www.vb-audio.com/Voicemeeter/
Voicemeeter is distributed as Donationware.
With this software the microphone input signal can be transferred digitally (streamed) from the Client to the Master and can be understood there by FS2Crew as default microphone input signal. In contrast to the solution with the splitter cable, the transmission quality is very good.
I am very satisfied with it and after searching for this solution for so long I would like to share my settings.
Cheers Stefan

As an appendix I have added a short description how I set it up on both my PCs.

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My settings
1.    Downloaded Voicemeeter by VB-Audio Software (Copyright V.Burel ©1998-2018)
https://www.vb-audio.com/Voicemeeter/
Voicemeeter is distributed as Donationware

2.    Installed Voicemeeter on client and master

3.    On Client (the PC where P3D is not installed)
Sound Playback

 
Sound Recording

4.    Voicemeeter settings on client

Voicemeeter VBAN on Client

5.    On Master (the PC where P3D is installed)
Sound Playback

Sound Recording

6.    Voicemeeter settings on Master

VBAN settings on Master

That's all. If you now speak into the microphone that is connected to the client, you can see a level swing at the level "B virtual out" on the Master.
For further information please look in the voicemeeter manual.

 

Edited July 13, 20196 yr by Stefan01

Hello Stefan,

For years I have used a speaker-out on the client to mic-in on the FS PC with vPilot on the client and FS2Crew on the sim.

Thanks for sharing!

I Agree totally with Stefan since I have been using voicemeeter for a long time. If you need more channels the voiemeeter potato has up to 5 inputs for a small price. I use Voicemeeter banana on my second pc and voicemeeter potato on the main pc.

Here is a link by Frugal on his thoughts

 

Ken