Hasn't this become a standard function in Windows 10? Thought I read about that. But if you have the room and the funds for it, a dual monitor setup will be much more useful than a virtual desktop can ever be. For most people/use-cases a dual monitor setup gives you a huge performance boost. With a triple monitor setup, results diminish rather quickly. Still, it can be useful. Having 4 or even more monitors is definitely possible, but then it is more of niche inside another niche.
The standard functionality does not fulfill your needs? The only one I know about (and barely worked with) was: Dexpot. Then my boss decided to splurge on making each workstation a dual monitor setup. Haven't looked back at Virtual Desktops ever since. That was more than 10 years ago. So, unless you absolutely must have only one monitor on your desk, spend money on a new or second hand monitor. You'll get for the price of software license a second hand monitor. Doesn't have to be the same size or resolution either.
Although having 2 same size/same resolution/same height (stand) monitors is preferable, it is not a necessity. Any second monitor is better than a virtual desktop. Always.
There is an alternative way of seeing/testing if a second monitor is for you or not. This does require a separate computer or laptop on the same LAN network next to each other. Or at least in eye-shot.
Software like: Barrier
an open source alternative for the nowadays way too expensive Synergy
can be used to share keyboard/mouse/clipboard controls between computers that use these software.
Your main computer acts as the server, the others as clients. When the software is enabled, you can use one and the same keyboard/mouse to control all the connected computers. And yes, you can hook up Windows, Macs and Linux computers all together like this.
If you have the hardware laying around, it is an easy download/configuration and test to see if a dual monitor setup is for you, without spending any extra money. And if you are impressed by what such a setup helps you forward, fitting a second monitor to one computer is even better.