ZMA Controller described a TCAS RA event involving an IFR departure from EYW and an unknown VFR aircraft immediately above the IFR; the reporter claiming that Center Controllers should not be assigned to work approach control functions during busy/complex traffic periods.

Narrative

During this session; ZMA had taken control of Navy Key West Approaches Airspace. In my opinion this already creates an unsafe condition because as Center Controllers in the West Side Coastal area; we have virtually zero experience working heavy traffic into an airport with an operational Control Tower. We are not Approach Controllers; and suddenly taking responsibility for an Approach Control during a busy weekend; compounded with severe weather which had been impacting the area all week; is simply a recipe for disaster. Aircraft Y was a VFR flight about 15 nm East of EYW; inbound for EYW at 2;500'. EYW Tower requested; and was given; a release off of EYW for Aircraft X off Runway 09 to maintain runway heading and climb to 2;000'. Shortly thereafter; Aircraft X checked in on frequency and was RADAR identified by my trainee. At the time that Aircraft X was tracking up; we failed to differentiate that there were actually 2 targets virtually on top of each other just East of EYW. One was the Aircraft X and the other was an untracked VFR squawking 1200. At first glance; I thought that the RADAR was simply trying to acquire the target. We called the Aircraft Y traffic to the Aircraft X; stating that traffic was 12 o'clock; 10 miles opposite direction at 2;500 and climbed Aircraft X to get on top of Aircraft Y which had started a VFR descent out of 2;500 into EYW. It was at this time that I noticed there actually was a 1200 code VFR target virtually on top of the Aircraft X with both altitudes indicating 2;000. This was easily the closest that I have ever seen two targets in my career. I attempted to call the traffic to Aircraft X and did not receive a response at first. After re-establishing communications; Aircraft X advised us that they had responded to an RA. Several minutes later the unknown VFR target called us for flight following. At this time we discovered that the identity of this aircraft was Aircraft Y; that had also departed from EYW and was looking for flight following. There were 2 main contributing factors in this incident. First is the unfamiliarity of ZMA Center Controllers with working Navy Key West Approach airspace during a busy weekend. I believe that if Approach Control had been working; they would have been watching the airspace on a much smaller scale; allowing them to more easily distinguish between the different targets; without the additional distractions of feeding MIA and FLL arrivals into Miami approach; working MTH arrivals; and dealing with significant severe weather in the sector; all during a busy weekend. In my opinion; allowing this approach closure compromises safety. Second; although I understand that it is perfectly legal to clear a VFR for take off; followed by a [higher performance] IFR; it seems to me that there should be at a minimum some sort of traffic call or additional separation to prevent the aircraft from getting too close.