As mentioned, all kinds of things change as we get into what we each call "reasonable"! For some the viewing of both tanks is not good, For others it will be the difficulty of working when two are stacked. If they are on the same stand one is going to be prone to being too high while the other is too low, so we all have to make decisions on those aspects balanced against floor space available. The weight is not often a real problem as tanks are often larger than the combined weight involved here.
For design, about the only thing required to set the second under the larger is to add upright solid wood support at the corners of the smaller.
Perhaps centered under the upper or it might be nice to have it offset to one side, leaving a bit of space for a door on the other end?
Lots of personal choices balanced against what work we can each do in the way of experience and tools/workspace, etc. The work is simple but the decisions can be hard!!
I have floor space and would go for stacking and extend the stand long enough to fit two canisters and equipment in one compartment at the end but I also don't like to fight for space to work in the tanks, so almost always use separate stands, rather than dealing with ladders/stools for cleaning. .