If you want to use more than a single monitor on Microsoft Flight Simulator to get different views on each monitor (like your cockpit view shown on one monitor, while a wing-view is displayed on another monitor) then read on…
If your graphics card can support multiple monitors, this is a fairly easy task. You can Google and find how to do the necessary configurations… But don't worry if you don't have a decent card. There is an alternative.
If you have two average-spec computers and a small home network, then the best way is to run FS on both computers and share all the flight data (position / weather / instruments etc.) over the network. Basically, your flight will be re-generated on the other(s), using the information transmitted from the "server" side.
WideFS is a popular solution. That's fine if you are going to purchase this software. (It's also useful for other things) But, if you are after a free and practical solution then you can definitely try RanaInside's flight manager. It's a freeware that lets you manage your original and add-on aircraft, and make many performance tweaks for your flight simulator. Additionally, it lets you connect multiple running copies of FS, which is more relevant for our task.
Download and install it on both (or on as many as you like) computers. Within the program, assign your (primary) computer on which you will play the game as the "server", and select the other computer(s) as "clients" on which another instance of Flight Manager is already running. Note that, you have to specify the IP addresses of each computer you are connecting from / to.
Before hitting the "connect" button, make sure FS2004 is running on each computer, and a certain flight (the default is fine) is loaded (yes on each FS!). Press 'y' to toggle between 'slew' and normal mode, if necessary.
If you are not entirely satisfied with the regeneration/refresh rate of your flight on the secondary computers, you can specify and lower the time interval determining how often network packages are sent. Make sure you won't blow up your router by reducing it excessively.
Fasten your belt and enjoy your flight 😉