Installation method is greatly appreciated on all support requests.
Easiest method is to set up the "secondary" dialer to link to a phone on the primary dialer. Then you can set up the phone as a remote agent by using the dialplan number. You can register by placing a Carrier entry in the secondary dialer to register to the phone entry in the primary dialer.
However, if you require the ability to dial a specific agent on the secondary dialer, you would need to set up both as Carriers and interlink them. Then you would use a dialplan entry in the carrier setting of the primary dialer to "capture" the call to that carrier entry and then generate a call on the secondary dialer to that specific agent.
We used to set this up all the time for clients who had both Vicidial and FreePBX. Works quite nicely.
IAX or SIP will both work for this.