> “Mixing sleep and study in the same location sends mixed signals to the brain,” Carter wrote in an email. “It does not know which it is supposed to be focusing on. This leads to less effective studying (and) learning, and it makes it harder to fall asleep when you want to.”

This may have to do in the context of Pavlovian Conditioning. 

Pahlov found he could trigger a dog's salivating reflex to the ring of a bell, by ringing a bell whenever the dog was fed over a period of time. (McLeod, S. A. (2018, Oct 08). Pavlov's dogs. Simply Psychology. https://www.simplypsychology.org/pavlov.html)

The theory proposed in the article is suggesting something similar: to condition your body to associate the bed with sleeping. So that, just as the sound of a bell makes the dog salivate, a bed where you regularly sleep in will also make you automatically feel sleepy when you lie down on it and thus help you fall asleep more easily.

But this can only happen if sleeping is the only activity that you do on a bed.

If you do other activities, and your brain find these more stimulating, than the bed can become associated with that feeling and trigger it. 

So for example, if you play video games in your bed repeatedly over a period of time, lying on the bed may automatically remind you of video games and get you all stimulated. Obviously a stimulated brain will find it difficult to fall asleep. And you may then go on to some other activity instead of sleeping, without realising why you couldn't sleep. Over a period of time, this kind of cycle (not being able to sleep and doing something else till you feel sleepy) could develop into a sleep order. 

> So my question is, can I do these other things on a bed on which I do not sleep in (in another room)?

Theoretically, you can. In effect, you can try to condition yourself to associate one environment with your sleeping activity and another for studying. You just have to ensure that the environment is different has enough differences (maybe use a different mattress on the bed, a bed sheet with a different texture, a bed placed in a different direction, a light source at a very different location etc. etc.). 

Once you've differentiated between the two environments, try to keep both unchanged as it is for a period of time.

> Contrary to what the article says, I just don't want to study on a desk, I find it very hard to focus (eg. in school) because my body position from sitting on a desk is not ideal/comfortable for me. 

Your body is telling you something when it protests (through aches and pains) when you sit for long hours on a desk. It is because you are not taking the right amount of breaks and hydrating and fueling yourself at regular interval. 

And make sure your chair and desk are ergonomically designed. 

In general, follow the guidelines as in using a computer. And if you use a computer install a software like Workrave (Windows) or TimeOut (Mac) that will force you to take a break regularly. The difference to the body on taking these micro-breaks and short-breaks can really be surprising.