How can I send a command to two separate screen sessions simultaneously?

Question

I'm monitoring two measurement devices (usbserial1 and usbserial2) over RS232 (via an USB adapter). I do this in the following way:

1. I open two terminal windows.

2. In one window I write the following commands:

   mkdir one
   cd one
   screen -L /dev/tty.usbserial1 9600

3. In the other window I write:

   mkdir two
   cd two
   screen -L /dev/tty.usbserial2 115200

Now I want to send the date command to both the screen sessions at the same time. How do I do that?
This answer describes how I would do it for one session, but I want to do it for two sessions simultaneously.