A ZMA Controller described a near conflict when communication with the aircraft involved was intermittent because of a site outage problem.

Narrative

I relieved the previous controller and was adjusting the speeds on the JUNUR/FLIPR arrivals. At the time that I took the sector; Aircraft X was South of URSUS and Aircraft Y was not on the scope yet. The frequency 135.6 was on the Miami site because the ZBV site was out of service. Using the Miami site makes it harder to hear the airplanes at low altitudes due to the distance from the site. I noticed the two aircraft were coming together but I was not talking to either aircraft. I then tried to relay trough a TPA flight and I asked him to instruct Aircraft X to descend to 070 immediately. I also attempted to have the same pilot relay for Aircraft Y and have him turn right 30 degrees; but there was some confusion as to the control instruction. By that point Aircraft X had responded quickly by beginning the descent when asked and was out of 082 for 070 and there was no need for Aircraft Y to turn. The airplanes did not lose minimum separation but it was an unsafe situation as well as scary. Once Aircraft X leveled at 070; he requested a climb to 090 due to moderate turbulence. He requested such through the TPA flight that was relaying. Once the airplane was within range; I asked if everything was OK and if any one got hurt when we asked him to descend. He said that everything was fine it was just the moderate turbulence at 070 that he did not like. He then proceeded to cancel IFR and proceed to the airport VFR. I never did speak with Aircraft Y. Should not approve two airplanes at the same altitude on crossing routes especially when the frequencies are not working properly.