Question about Virtual-Um implementation

Hello all, We trying to bring up set of BTS with virtual Um with different configurations: 1 BTS -> 1 TRX -> 1 MS works fine as expected 1 BTS -> 1 TRX -> 2 MS works fine as well But I see some unpredictable behaviour when I have 1 BTS -> 2 TRX(872,873) -> 1 MS MS can attach on one ARFCN (872) but when it tries to attach on 873 looks like PRACH does not reach BTS and I do not see Channel Request on BSC side. So, the question is it workable configuration: Several TRX to 1 MS with different power level to allow MS to choose a best cell to attach? I have just one idea: to have several virtphy instances for MS connected to different UDP ports. Thank you Vania