It's those defaults that are messing up my head again.
If I take sword-and-shield(SAS) 8, then I get mass-weapon-and-shield(MWAS) at 7. If I then improve MWAS by one I get 8 there too.
If I then during play improve my SAS to 9, does MWAS improve too, or does it count as independent of SAS(i.e. it doesn't improve when SAS do)?
Or is the default-thing something you only bother about at chargen?
First thing: Defaults max out at 6. You can never have a 7 on a defaulted proficiency.
As for the meat of your question, this is my take on it...
Default scores only raise with the base score so long as they're default only. To use your example..
SAS defaults to MWAS at a -1. If I have SAS at 8, I'll have MWAS at 6 (max default) by default. If I later raise MWAS to make it a regular proficiency, then afterward raise SAS, MWAS does not go up again, as it is based on it's own merits. That's how I'd do it.
Alternately, you could have the secondary proficiency raise so long as it is under 6 by default.