How to download but not install OSX Mountain Lion on Mavericks

Question

Hi all
I would like to download but not install OSX Mountain Lion on my MBP which is currently running OSX Mavericks.
This however seems impossible. I think OSX Mavericks were already installed on my MBP and therefore it seems "impossible" to download it.
Now, why would I to do this? The answer is quite reasonable, due to certain demands related to an application portfolio I need Mountain Lion, because the portfolio is not upgraded to run on Mavericks. And, it is more reasonable to downgrade a version of OSX, rather waiting for the portfolio upgrade which can take years.
Yes, I need to downgrade my secondary MBP which also came with preinstalled Mavericks. This can be done, if I only could download but not install Moutain Lion.
It should be easy enough; download Mountain Lion, make an installer and make a bootable thumb usb stick. Then erase Mavericks and install Mountain Lion.
I have previously bought Mountain Lion so it is accessible to me in the App Store, but whenever I try to hit the Download button, I get a message saying this can't be done, since I am running Mavericks.
This bugs me, it should be obvious users needs the flexibility to swap whatever OSX version they need. It is still software.
So, how can I download but not install Mountain Lion on my newer MBP that came with preinstalled Mavericks?
I would like to see answers like:
"In order to get this to work, you first have to disable the locking mechanism in [...] " instead of "No, this is impossible to do"
Regards

Answer 1

Thanks for your reply Eric
Can't help wondering thought, people are ok with this?
Basically Apple are saying: "Do not attempt to install an older version of OSX which came preinstalled. If so, expect multiple failures." Really?! Has it always been so? For me it's not like I'm going to install Snow Leopard on my Late 2013 Retina MBP, but I would expect some form of backward compatibility between the previous version and it successor. Apparently this is not possible, and how is this even acceptable? 
Personally, I am not going to downgrade, as if I had a choice, but I would and I expected however, the possibility to at least download the previous OSX version. What do you do if you have bought several applications, not necessarily online from App Store and buy yourself a new Mac with preinstalled Mavericks and after installing your "old" applications they fail to work properly? What do you do since downgrading apparently is not an option. So... again, what do you do? 
If you don't have friends with an older Mac, you could be facing serious problems. And I'm not talking about gaming, but serious business. 
I find this quite disturbing actually, and please tell me it's not so. There gotta be a way to do this?
I simply want to download the OS but I expect some registry values (like windows' registry) keeps preventing this.
It should be possible to simply download and not download and install in one operation, shouldn't it?
Regards