I understand that probably to preserve a realistic look the two parameters are linked, but the choice should be left to the artist if he/she wants to unlink (a check box for example to unlink) these two parameters "Bluesky density" and "Redsky decay". I'd like to tweak the blue and the decay separately.
Same applies under the tweak tab:
option to unlink Bluesky density colour and redsky decay colour.
It's not possible to unlink these parameters because they are simply different interpretations of the same internal parameter.
Although you can't change the density and density colour independently of the decay colour, you can change the bluesky additive amount and bluesky horizon colour. These provide the same level of flexibility that you would get from a system that only had redsky decay and bluesky additive colour (like Terragen Classic), except you now have the ability to control the combined result more precisely.