This doesn't answer your question, but it's another solution. Our bathrooms are small with limited storage, so our solution to limited storage is these delta commercial holders that recess into the wall, creating a space for the spare behind the one on the roll. It looks good.

I think those delta holders were only around $12 each, too.

My son only has use of one hand so the open-ended TP holder and reserve are ideal. This is my first bathroom reno in 23 years so I splurged on holders that match the clean lines of the other choices.

Ha! I'm using the reserve holder as my only tissue holder. We had a rather narrow space and no cabinet next to toilet. The "reserve tissue" will be in the linen tower about 4 feet away if there happens to be an emergency. :-)

I've never seen a reserve holder in use in real life. Could you install it on other side of toilet?

I don't like anything near the stool because of the fine mist it puts out when flushed. I keep my extra rolls of tissue paper in the vanity which is an easy reach.

I use an upright holder like your "reserve" holder. I keep all spare TP in the vanity drawer next to the toilet. Note that my grab bar is too far back, but I had to place it there due to the door frame.

My vanity is not in easy reach for anyone other than Disney's Mrs. Incredible aka Elastigirl. For the last 23 years I have had a roll on the floor, which I chose to remedy as we renovate; enter the purchase of the reserve holder. With two adults, two teenagers, and a 1.5 bath house, this full bath is highly trafficked. Since I appear to be entering rarely traveled territory with a reserve holder, I will take a stab at placement and post pics :-)

I really like my upright holder. Have you thought of just putting two of the upright holders side-by-side, forgoing the sideways toilet paper holder. In my experience I always pull off the roll from those sideways holders. I made sure I had blocking behind the wall for the mounting brackets. So that they would be less likely to be twisted off the wall with pressure.

I have blocking in the wall for the TP holder, and that is actually why I asked about placement of the reserve, as I am adding blocking for that too. We have had a horizontally installed open ended toilet paper holder for years with no issue. Perhaps it's the placement of our holder on the wall to the right of the toilet (rather than the back wall), combined with our favored under vs. over hanging of the roll, but the roll does not get pulled off. I have already purchased the holder and the reserve, and will be using one of each.

Yes, I bet it is the side placements for those side opened holders. If I had one I would have the opening facing away from the stool, (as I would mount it to the side of the toilet). This way, the force of the pull would keep the roll on the holder.

I wasn't sure about the upright one but I got it anyway cause I thought I might like it. I really like it. I don't have young kids so the single post attachment hasn't been a problem. If I had little ones I'd want a 2 sided mounted holder.