That's a different thing In the post you are quoting, @max83 asked about using a self-compiled new kernel with an existing Sailfish installation. Answer to that is, yes, just put modules and kernel to ALT_OS partition (normally the mmcblk0p4 partition) and make a new kernel menu line to "Sailfish" section in ubiboot.conf. What you are talking about here, I suppose, is having 2 different Sailfish versions on the device at the same time, on different partitions? And the answer to that, too is "of course it is possible" If you want to do that, you need of course partitions for both your installations, and the preparation is similar as for ALT_OS, untarring the images to partitions and then making ubiboot configuration entries for the partitions. (Additionally, you could change the menu graphics (boot icons) so that you'd have for example "sailfish r3" and "sailfish r4" in your top menu...)