Thanks guys, that was all so informative! :goodjob:
I actually realised a thing or two about my whole plan, I think was trying to do things the hard way any ways. But I′ll reply in order:

To make a second installation, just use mTES4. [EDIT] Sorry, Psymon is right.
You should not create clones from an already modified and used installation. See the guide I wrote in the post below.
You are right though that mTES4 requires you to have both copies installed on the same drive.
Technically, you can't install Oblivion twice. What mTES4 does for you is replace your game folders with the other copy.
The reason that mTES4 requires the copies to be in the same place on the same drive is because if you were to swap them from different drives it would require
copying/moving your existing game folders to your other drive (which will take about 5 minutes for a vanilla install),
then copying/moving your duplicate game folders back to the other drives (taking another 5 minutes).
If you don't have enough space on your existing drive, maybe you could move your current Oblivion install onto your new drive when you get it?
On a side note:
In my personal opinion, SSD's are not worth the extra cash unless you have a specific, optimized use for (a small) one. They are still a relatively new technology and pricey.
One great use of SSD's would be to put your windows installation onto it, your windows swap file or disk caches for things like Photoshop.
If you plan to use your SSD for general purposes while your system is still installed on an HDD, I'm afraid you won't see the speed improvement you might expect because windows itself will slow you down.
Don't get me wrong. SSD is really great but it's advantages only truly shine when you install your system on one.
Either way, SSD's are alot more robust and require far less power. The best part of all is that they never have to be defragmented though!
SSD's will always run at peak performance, no matter how full it is or organized the data is. HDD's get slower the more stuff you put on them and the more disorganized the data becomes.
[EDIT]
I just reread your post after replying and realized you already have an SSD. My advice is still applicable to you and anyone else who might be interested in buying an SSD though, so I won't strike any text.
After reading this and the guide and Psymon′s replies, I realised I′m not even trying to manage multiple installations - I just want to be able to launch my old setup (for modding, doesn′t EVER need to change, saves and ini settings are meaningless, etc...), AND be able to install another copy, on the new drive, for playing.
So what I would be managing with this tool, includes ONLY the playing copy. That′s the one I′m adding mods for and playing, the other one needs to exist and needs to be launchable only for the purposes of launching CS and occasionally loading SOME old save to test some script.
So I′ll simply install the game on the new drive. Period. Because, I can STILL launch the old copy directly from the folder. All the ini settings and saves can just as well be "owned" by the playing version.
About SSD:sI bought a 60Gb drive, just to play Oblivion (at least for now). So that files that are loaded in-game load faster. Also put my page file on it, cause some stuff will end up coming from there anyways.
I got to test this with my current (modding) installation, by simply copying the folder over and launching from the folder. And the difference is STAGGERING. It′s just so damn SMOOTH. :celebration:
My current installation was running from two 7200rpm raid 0 HDDs, and measuring the difference with HdTune gives 2-3 times average sustained reading speed. I can′t measure seek times or random access times with the free version, but those should be up to 10 times higher (especially with that old HDD pretty crammed).
So it′s not a miracle the difference shows, since 2006 my CPU, GPU, memory and GPU-memory have about quadrupled in power or amount, still this was the biggest individual difference I got from upgrading. It was also the cheapest upgrade (150e).

You can get hybrid drives now. They seem very promising. They are like regular HDD's but they have a tiny builtin SSD that is used to automatically optimize storage of small and frequently used files.
I actually considered buying one of those, then read some reviews. The only time the hybrid (7200rpm) drive got up to VelociRaptor (10000rpm) speed was when they repeated a pre-recorded set of drive actions - and eliminated the first test from the average. This way the SSD buffer was already holding the needed data. In all other testing, it got [censored] by the 10000rpm drive.
So at least for now - especially when it comes to reading thousands and thousands of different and random game files from the HDD - the hybrid drives and pretty much the same as using non-hybrid drives, the only thing that matters is the traditional speed of the drive. So actually it seems that those are the ones with no good excuse for the extra price - at least not for gaming purposes.

EDIT: The second quote was a wrong one...fixed..