how to use 2 usrp b210 to receive synchronously

[james jordan]

Hi all,
i have 2 usrp b210 and i want to make 4 channel rx system. i make a uhd source 
and config mboard num 2 and channel count 4. but when i run the programme it 
says "RuntimeError: LookupError: IndexError: multi_usrp: RX channel 2 out of 
range for configured RX frontends"
so how to set the source block for 2 b210?