Since you are doing this for a FD it needs to be done correctly. Done poorly you will have distortion and paging tones will be distorted and fail, the audio poor and CTCSS may even suffer. You need to have exactly same transmitters on both ends. A good place to research the proper equipment is www.simulcastsolutions.com where you can research proven solutions,
Ed OConner has decades of experience and his site has a lot of application notes and studies.
You will need someone to perform a simulcast coverage map if the proposed system. A proper map will take into account the differential time delay between sites and will highlight areas of potential distortion. You might get away without this step on a two site system if the sites are close to each other, however there will be time delay interference distortion and you need to know where it occurs duruping the planning stage.
Someone brought up the RTCM. It is an attractive low cost alternative, however it has a design bug with resided to the 9.6 MHz clock. Fixing this is not simple. The RTCM is really not suitable for public safety instaklations.
You will need a voter for simulcast and the JPS SNV12 is the only product I would recommen at this time.
Linking sites requires a digital circuit that is low jitter and has stable latency. The internet won't work in this case.
RFI-EMI-GUY