First, you don't want your Citrix in full-screen. So go up to your Citrix Settings bar up at the top, and make sure its not on Full Screen. Now you should be able to see all four borders of your citrix session.
Next, grab the edge nearest to the other monitor you want to use and drag it across both. Similar to this:
If it doesn't automatically fill both screens, then go back to the Citrix Settings bar at the top and click Full Screen.
Troubleshooting tip: If both screens are not running the same scale % then it may not work and only stick to one screen.
You can fix this or check by doing the following.
1: Right click on the desktop background and select "Display settings".
2: The number on the screen represent your monitor, you can click on them to select a monitor in which you wish to change or view settings for.
3: Scroll down and you will find the scale and layout that can vary depending on your monitor's resolution. All we need to make sure is that they both have the same one such as Monitor 1: 150% and Monitor 2: 150%. It can be whatever size you want so long as they both run the same percentage.