How to control two UHD USRP SDR cards simultaneously in GNU Radio?

I have the attached simple flow graph in gnu-radio. I'm using two b200 mini SDR cards and both are connected to the computer. I want gnu-radio to run them both at the same time and I want to be able to compare their received signals at the same time. Surprisingly, gnu-radio runs only one SDR card (with no errors) even though the graph clearly shows two. I can completely detach one of them and the graph runs with no errors. This of course not what I want because the graph seems to use one SDR card instead of the two.

I looked online for an example of using multiple SDR cards at the开发者_运维百科 same time but didn't find anything helpful. I found a question with a similar flow graph on Stack overflow but wasn't able to comprehend the given answer.

I also found this question on stack overflow where I had two USRP sources each with Num Mbords = 1, and Num Channels = 1, but I had no luck.

Any help is appreciated, and thank you very much.