With two pre-made UI's, there are complex issues with using them on different characters - namely, the fact that you need both original sets of lua files from the WTF folders.
One of the more reasonable methods involves using two WTF folders and swapping them out. Other reasonable methods involve
creating a sandbox installation - giving yourself another copy of WoW, without duplicating the game's 25+ GB hard drive requirements.
There are "unreasonable" methods of doing this that are far more complicated, however, and they pretty much assume you're an experienced geek and don't mind having to work on lots of fiddly little details. :P
Well, only 1 that I can think of offhand.
It involves first installing one of the two UI's, as well as Reflux. Then you set up Reflux to emulate profiles for every single addon (in BOTH UI's!) that doesn't adhere to a per-character database. Then you specifically tell Reflux to snapshot your current setup to a new profile name - UI1 or DPSUI or something. After that, quit WoW to the desktop.
Then you take all the files from the WTF folder in the 2nd UI, and make them load as an addon with a name like "zzzzzzzzzzz_wtf", just to make sure it loads dead last. This will set all the addons to the profiles from the second ui. Now, snapshot your current setup to another new profile - "UI2" or "TankUI" or whatever you wanna call it. Now, quit out of WoW and
delete the "zzzzzzzzzz_wtf" addon.
It SHOULD work, at least in theory, by using profiles (and Reflux's profile emulation) to do it - but it's a GIGANTIC pain in the ass. Also, the 2 paragraph explanation above is just an outline - there's a significant amount of details I didn't cover. And no, I'm not going into more detail - the amount of ui-pack specific "fiddly bits" involved makes me shudder.