Unfortunately, in a shared hosting environment, not really.

If you go to yourdomain.com/webmail and login, you will always have the choice of one webmail interface, or the other. Some users prefer Horde (my favorite when I am using a desktop or laptop) and some like squirrelmail (my favorite when I am using a mobile device like a PDA to access webmail)

If you have a dedicated server with us, you can definitely choose which webmail package to offer at the domain.com/webail interface.

At one point in time there were some php scripts available to bypass the main mail page and they would allow direct access to one webmail program or the other.

I have not seen any in a while that actually work in a shared hosting environment.

Some browsers will support direct linkage to horde webmail (I believe FireFox will) if you use this link:

https://youdomain.com:2096/horde/login.php

Internet explorer will not in most cases as they have patched some security issues using this type of login. If anyone finds a way to do this feel free to post on it.