Yes – if you try to install 1.7 with 2.1 installed it’ll tell you just that. You can them partially co-exist, by uninstalling 2.1, installing 1.7 and then downloading and using AxCrypt 2 portable / standalone. The version 2 won’t integrate with Explorer, but it can be run standalone.
If I may ask – why do you want to? We know there’s a lot of changes from 1.7 to 2.1, but once you get used to them we feel they are for the better! If you could tell me what your concerns are, I may be able to help or explain how you should be using version 2 to get the benefits it offers vs. version 1.