The gr-osmosdr source can be configured for two channels, which should give you two streams. You cant actually have two separate gr-osmosdr sources for one single LimeSDR because the channel 0 and channel 1 streams are physically aligned as they flow from the lms7 to the fpga to the usb…
Multiple gr-osmosdr sources can be used for multiple LimeSDRs. Use the serial from the LimeUtil --find or SoapySDRUtil --find to specify a particular device. driver=lime,soapy=0,serial=1234
So four receive channels for two LimeSDR devices would involve two gr-osmosdr sources, each configured for two channels, and different serial numbers.