Actually, there was a fairly good explanation posted in the forum (somewhere). The problem is that it can be difficult to search the forum, and even more difficult to find anything.
Although this is not supported by Sparx, it should get you by.
Install 7.1 normally.
Copy the installation folder for 7.1 [The EA folder if you used the defaults.] to something convenient, probably as a sibling folder tree to the original.
Install 7.5 normally. [That is, use the defaults, or the same settings you customized earlier.] Provide the default answers to any dialogs relating to prior versions.
Now you should have a functioning EA 7.5 installation.
Create a shortcut for the copy of EA 7.1 that you created earlier. Name it something convenient, and remember to set the directory it runs from the the copied folder tree (if necessary). Put a copy of the shortcut in each place you might want to run EA from (desktop, quick start, etc.).
Now you should be able to run the version you prefer.
Here are some things to be aware of, in no particular order.
You might get the EA 7.5 help file when you press F1 in EA 7.1. If that happens remember that you could see help for features not yet available.
Both versions will share a single copy of registry settings. This means that some things that you set in (say) 7.1 will be reflected in 7.5. This is not generally a problem as long as you know it is happening.
You may have some issues with loading and unloading of MDG Technology files, particularly those that are specific to one version (there will be some over time). Tweaking the MDG Technology settings in each version might not solve these problems (due to shared registry settings). Play around a bit and see what happens.
Once again, remember that running two versions of EA side by side is not supported by Sparx. If you run into problems don't panic and send angry messages to Sparx Support. Try removing the older installation and reinstalling the newer one (to get a clean installation and registry tree) to see if the problem is real. Then try to diagnose and solve it.
HTH, David