Probably been alot of folks think of this before me as I ain't the sharpest arrow in the quiver! In my hunting area, due to terrain and cover, being 18' to 20' up is optimal. I have started to prefer a ladderstand in my "golden" years, but around here you can buy 12'ers for $49-$59 and 15'/16'ers for $59 tp $79. The 20'+ ladders usually run around $175-$250!!!!
Like I said, this may be an old idea but if it saves anyone a few bucks, it's worth posting! Buy 2-15'/16' and one 12' ladderstand and use the two bottom of the 12' stand to make the 15'/16'ers into 20'+/- stands. I place the section with the support (goes from ladder to tree) up one section higher and just take a couple of climbing sticks to access the support to tie it off to the tree.
As far as the safety of this is concerned (structural) I emailed 3 different companies that manufacture laddstands and the 2 that replied stated the the ladder sections on all of their ladderstands are interchangable and are manufactured to the same specifications.
Using the prices in my area, I can end up with two approx. 20' ladderstands for less than $190 w/tax. I use the remaining section as a practice platform. Only a little over 5' tall, but I se it up to shoot down hill so I am getting my 18'-20' height.
Hope this saves some folks some time and money! Have a safe season! Mike