Reddit mentions of Smart Living Company Rock Design Gift Indoor Tabletop Desktop Water Fountain

It's less common because it's not something you would really expect to full size growing naturally. That said I think it could be a really awesome project but you should take that into consideration. Just because they might not look natural in a forest type layout doesn't mean you can't incorporate them into the same piece.

While implied water is what i've seen most, I think if you can pull off the real thing it will be very impressive. There's all sorts of desktop waterfalls and fountain type things like this http://www.amazon.com/Unique-Gifts-S34807-Tabletop-Fountain/dp/B000DZM1ZE/ref=pd_sxp_grid_i_2_2 that you might be able to build on to and get the desired effect.

I could picture something along the lines of a fountain similar to that (without the giant marble) set into a case or box fashioned to look like a continuation of the rocks, with a model temple or shrine at the top and the trees on either side above or below. This gets the flowing water and both trees in one piece, and it gets rid of the suggestion that it's some forest they're both growing in and adds a more deliberate look to their placements.

Either way that was just an idea. Look forward to seeing what you come up with!