If the antennas of modules are too close to each other, it will bring EMI interference within the antennas, which will result in the decline in the signal quality and the raise of BER, and cause long time connection or connection failure issue. It is suggested that the distance between each antenna should be more than 20cm.
If many modules are working simultaneously, it could cause some modules unable to register to network due to lack of base station resources. It is recommended to use QuecScan/QuecLock function to lock different modules at different frequencies. For Details, you can refer to <GSM_QuecCell_AT_Commands_Manual>. Also, you can use SIM cards of different operators if possible.