With the advent of media-video/avidemux-2.6.2 and its split off media-libs/avidemux-core I can't install avidemux:2 and avidemux:2.6 simultaneously anymore, i.e. slotting doesn't work anymore. Thanks for looking into it, Helmut.
Please realize that any versions before 2.6.2 are masked since they have security issues and are broken which is likely why you can't install them (you would need to explicitly unmask them), the current version is still half broken but I am in the progress of fixing that; the slotting is just fine, as you can see in `eshowkw media-video/avidemux`.