WHY THIS GUIDE? Recently, I had troubles myself while trying to test one of my plugins. It required at least two people to test because it was a sort of arena system. It was hard to find people to test with me (you're literally asking to spend their spare time) and the development of the plugin didn't advance in the way I wanted. That's why I recently asked a question over here on how to fix this. Since I didn't find any useful tutorials specifically for this issue, I decided to write one myself so others can benefit as well from this. WHAT ARE WE TRYING TO ACHIEVE? We want to be able to have multiple instances of Minecraft, preferably the same version, to run on one screen, so we can easily switch between two instances for testing purposes. Doing this by creating another instance in another version through the Minecraft launcher will not work, as only one instance per account can join a server. E.g. We started version 1.13.2 with the Minecraft Launcher and joined Hypixel for example. Now we go back to the Minecraft Launcher and create another instance with version 1.12.2. We can't join Hypixel with this version because we joined it already with another version of the same name. This tutorial will only focus on the testing aspect of this issue. Doing this on a public server is dangerous, as we are going to bypass certain checks. If you still do this on a public server, hacked accounts and players with VPNs can join as well and do horrible things to your server. The bright side of this method is that you don’t need one single valid Minecraft account, but it is supported to use one. STEPS TO REPRODUCE I am going to try to explain everything as detailed as possible. If you have remarks, suggestions or questions, do not hesitate to ask them! (1) Go to the official MultiMC download page and download the version according to your computer's software. E.g. if you have a computer with Windows installed on it, you should choose the 32/64 bit Windows version. (2) Open MultiMC and go through the default download steps. This should be very familiar and is really straightforward. (3) When you downloaded and set up MultiMC, open it. You now should see this tab, but without any instances (or a similar one if you have a newer version than I have): Spoiler: Image (3b) This step is only necessary if you would like to play with one valid account and one offline account. If this is not necessary (so only offline accounts is good enough for you) you can skip to step 4, but I highly recommend adding at least one valid account. In the upper right corner, you'll see the 'Profiles' menu with a steve icon. Click it and choose 'Manage Accounts'. Then click the ‘Add’ button to add an account. Spoiler: Image Now simply enter your account information like you would on the normal Minecraft Launcher. Then click close. (4) Now we have to add an instance to our MultiMC manager. Simply click ‘Add instance’ at the upper left corner (with the file as the icon) to start this process. It’s a really straightforward thing to do: just enter the name of your instance, the group (which is optional) and the version. If you want to learn how to import custom versions, I advise you to search for it on Google. There are plenty of tutorials out there and it is really easy to do. Spoiler: Image (5) After that, select this newly created instance and press ‘Copy instance’ to make a copy. Just enter a new name and version and you’re done. I made my main ‘Vanilla’ and my other one ‘Vanilla ALT’, so I know which one will be my alt account. If you want 3 accounts to run at the same time, you should copy it twice, so that you have one instance for every game you want to open at the same time. Spoiler: Image (6) After you did this, you can now open your instances by selecting the instance you just created and do the following steps according to the things you did earlier: If you logged in with a valid Minecraft account earlier, select your main instance and press ‘Launch’. Then press your alt instance and press ‘Play offline’ and choose a name different to the name you entered earlier. If you didn’t log in with a valid Minecraft account earlier, select your main instance and press ‘Launch offline’ and choose a name. Then press your alt instance and press ‘Launch offline’ as well and choose a name different to the name you entered earlier. (7) The last step is going to your server files and opening the ‘server.properties’ file. Go to ‘online-mode’ and set it to ‘false’. Make sure to save the file. Spoiler: Image (8) Restart your server and if everything went well, you can now join your server, even with invalid accounts. I repeat: only use this on your own test server, as others can make use of this! SUGGESTIONS, REQUESTS, QUESTIONS... If you have any questions, suggestions or requests, do not hesitate to write a comment under this thread. I am not a native English speaker, so if you find any grammar mistakes, you can tell them in PM. As you see, I am not a fluent tutorial writer and this is actually the first time I made one, so if you have a better explanation or if you actually find an error in the tutorial itself, please let me know as well. I also didn't know for sure if this was in the correct sections. This was the only section that I could find where you can create 'Resource' posts. Feel free to move it if I was wrong. I hope this tutorial was useful for you.