you could install both versions, but you have to do it this way. First install the main games from GOG in e.g. C:\EECHAll (not the way the GOG install recommends it). I do this with using the subfolders Aphavoc for EEAH and Cohokum for EECH, because this was the way the old original games from Razorworks had been installed.
This way it's also possible to use 1.15.2 Fix5 which is at the moment the last version with a working Havoc cockpit (but messyhead is working on a fix for this in 1.16.0) and this need to be installed this way for the gauges to work.
Now backup this install, so you have a clean installation at which you can now apply the different mod versions. After you have upgraded this install with a mod e.g. 1.15.4 rename it to EECHAll - 1.15.4 for example. Now copy your backup to C:\EECHAll again and do the next mod upgrade e.g. 1.16.0 Fix1. After you done this you could rename it to EECHAll - 1.16.0.
Now you have both versions on your HD and must simply rename the one you would like to use to C:\EECHAll. This way the registry entries always correspond to the install path. If you want to use 1.15.4 rename EECHAll - 1.15.4 to EECHAll and let EECHAll - 1.16.0 left. For using 1.16.0 rename EECHAll back to EECHAll 1.15.4 and EECHAll - 1.16.0 to EECHAll.
Sounds a bit cumbersome, but it isn't and works like a charm.
