I want to install an MSI twice

Virtualise, seriously

It is not possible to install the exact same MSI twice. You might be able to open the MSI and change the GUIDs to get it to install a second time. Be aware that this way there be dragons. Unless you are exceptionally lucky or the install/product is exceptionally simple, I can almost guarantee that not only will your second install not work, most likely you will break the first install too. I would highly recommend working out whatever issue makes you think it is necessary to use one box.

Have a look at what Scott Willeke's Less Msiérables (LessMSI) can do for you ...

http://lessmsi.activescott.com/

Use it to extract and change guids and/or contents of MSI, repackage and execute your 'install twice' need.