Awacs, remember that GSM is a TDM access method, so on the BCCH ARFCN, there will be 8 TS which are TDMed, and one of them is BCCH, and the remainging 7 is the TCH. All this is very abstract and its a bit difficult to understand. U can google GSM TDM frames and get some pictorial info so that u will understand it better(difficult to explain in words).
So if you go by this logic, 2 TRXs will have 2 ARFCNs, and they use a duplexer or something and radiate using the same antenna.
Other members can probably explain better:))