K&M 18881

I like these extensions so that you can adda 2nd keyboard on top but they do lack some adjustments. If you are fussy (like me) about exactly where the top keyboard sits then you may need to get get a bit physical with these and mangle them to suit your needs.
I ended up hacksawing off about an inch or 2 of the arms so the top board sat a little further back and added some new bolts as restraints to stop it falling off. It is a bit of a hassle to do that with a brand new product that you have just bought but there was no alternative for me. If there had been more adjustments possible I may not have had to do all that.

I bought the three tiers as a package. They are great for temporary studio setups. I've never gigged with them. Setting up all 3 tiers takes a while, but once configured it can be quickly collapsed without changing anything but the width. Much better than a multi-tier X-style stand, which I will never go back to. Not perfect, though: To keep stand parts from getting in the way of your playing, the tiers must be spaced 6-8" apart, and (vertically) shorter keyboards may need a height boost.
For studio users like myself, if you have 2 or 3 keyboards to keep stacked long-term, and you're willing to spend a bit more money and time, DIY solutions yield better results. There are a few good methods: Wood and construction adhesive; structural pipe connectors such as MakerPipe; or a combination, which is what I use now.