I'm not a frequent poster, but this site has been a great source of info. I moved recently and am setting up a hobby setup in a bedroom. I wanted to put a few pieces of gear on my desk, under my monitor. A 4u rack with 2u x 2u sitting side by side would fit perfectly (big Ikea desk). The problem is, I can't seem to find this product anywhere online. I can get individual 2u units, but I thought a combined unit would be a bit cleaner.

I'm in Portugal, in an apartment and have very little access to tools to modify an existing product. Any help would be appreciated.

Ask, and you shall receive. I think the price ridiculous, but this is exactly what you want.
https://www.audiorax.com/db2u

I would probably just buy two 2us. Do they really need to be attached to each other side-by-side?

That is a bit excessively priced. I had wanted a combo just so the two units didn't move around on my desk separately. If I had tools, I would order the rack hardware and build something myself but alas.

If there are rear rack rails, you can attach the two together with a couple of pieces of wood and bolts into the rack rails.

or just drill two holes in the mating side of each rack and attach with short bolts, sized no longer than the width of the rack rails.