We may need a bit more information from you to be able to solve this problem, but I can have a guess at a possible cause.

Have you hard coded the device ports for the two sensors? The first connected device will usually be something like /dev/ttyUSB0 or /dev/ttyACM0, the sensor you connect second will be numbered with a higher device path such as /dev/ttyUSB1. You may need to update the device paths for the second sensor from its default value for it to work.

The tidiest solution is to setup some udev rules on your system so that the devices will be detected on connection and symlinked to a clear and unique path. I can explain how to do this if this does turn out to be the source of your problem.

Hope this helps.