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 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.

I was able to fix the issue by specifying the device serial number in the USRP Source block.

1- In terminal type uhd_find_devices then get the serial number of your connected SDRs. Assume they are A1B2C3 and 3C2B1A.

2- Double click on the 1st UHD: USRP source block, then in the properties window, put "serial=A1B2C3", then click ok.

3- Double click on the 2st UHD: USRP source block, then in the properties window, put "serial=3C2B1A", then click ok.

In this way both SDR cards will be used.