A number of RealSimGear hardware items have built in video displays, such as the G1000, GNS430/530, and GTN650/750 units. Each of these units require a dedicated HDMI video connection to your PC, and once connected to your PC, need to be set up as "Extended Desktop" mode. (Note: RealSimGear does not supply HDMI video cables due to the variations in video connections on a PC).
As one can imagine, if your simulator is already running other displays, for instance the standard 3 display setup for external visuals, and one wants to add the G1000 package (for instance), your video card may simply not have enough video connections to support it all.
Well good news! There are a few inexpensive options out there to get a few more HDMI connections from your PC.
First note, be careful that you DO NOT use a SPLITTER. Instead, make sure you are using one of the options below. A "splitter" will give you 2 for 1 on screen support, but it will simply duplicate your images, not really want you want.
In many cases, your PC is using a stand-alone video card (such as a GTX 1060/1070/1080/2060/2070/2080 etc). However, in most cases, your motherboard ALSO has a small video card in it as well, known as the "onboard video". Typically this onboard card is disabled in your computer BIOS when a stand-alone card is installed, but it is possible to re-enable the on-board and use them to power more displays. Check with your PC maker on whether this might be possible on your system.
If your video card (on-board or stand-alone) has a DisplayPort connection, you can purchase device called an HDMI hub, such as THIS one, that allows you to split a single DisplayPort into up to 3 HDMI ports. However, this will not allow you to exceed the maximum number of connected displays, only provide you with more HDMI off a single DisplayPort.
If your computer has USB3 ports (the blue ones), you can use a USB3 > HDMI adapter such as THIS one. It will provide you dual HDMI ports (almost like a small external video card).
Or THIS one which provides a single HDMI connection
The same goes for USB-C ports, THIS USB-C > HDMI adapter will provide a single HDMI connection.