After looking around online it seems pretty easy to have multiple dropbox accounts running. All you have to do is change an environmental variable and then run dropbox. However, I've tried editing the .desktop file so the Exec line is changed from this:
Exec=dropbox start -i
which is the default, to this:
Exec=env "HOME\=/home/reg/.dropbox-alt" dropbox start -i
which from everything I have read should work. I've also tried all the variations of escaping and quoting like:
Exec=env HOME\=/home/reg/.dropbox-alt dropbox start -i
Exec=env "HOME=/home/reg/.dropbox-alt" dropbox start -i
Exec=env HOME=/home/reg/.dropbox-alt dropbox start -i
and nothing seems to launch dropbox. However if I try the same line in bash it tries to launch but falls short but that's only because dropbox is looking for a GUI. That being the case I would have thought that doing the above in the .desktop file would work but I get nothing at all happening.

I'm doing this without any dropbox instances running already so it cannot be that dropbox is looking for other instances and stopping itself from loading another instance.

If I try this in the .desktop file:
Exec=env dropbox start -i
It will launch dropbox but now it's the default instance which has no benefit.

Can anyone tell me what I'm missing to make this work?