What MonsterHPD said!
I used the 'one prog, one straight rate' springing for a couple reasons, only a few of which are relevant here.
The straight rate springs I had were pretty good, but I thought maybe I could get better compliance with a lower rate spring and raise the oil level to maintain bottoming resistance.
But then the trifecta: no softer linear springs on the shelf, lots of OEM progressive ones on the shelf, and at times I'm a really cheap and impatient bastard.
So one linear and one prog it was.
Worked great with a little fiddling.
It got me to where I wanted to go, but it's got some baggage.
Aggro is setting preload, depends on the topout springs.
If the topout springs are stiff enough, the forks want to be different free lengths unless they're preloaded enough to completely squash the topout springs.
So then the fork bottoms are misaligned and the axle really does not want to slide in or out.
This can lead to confusion as the axle went in/out fine before and that stuff didn't get changed.
So then hammers are being being picked up and that's often a bad approach for things that are binding.
If you're lucky the topout springs are soft and even at zero turns of preload they're squashed.
Since running into this issue more than once, along with the common 'axle now oval due to overtightened pinch bolts' syndrome, I check for a relatively free-sliding axle before I pick the wheel up and struggle to install.
I have a small pile of prog springs and it feels like a bit of a waste if they just sit there (tho I didn't manufacture these generally unsuitable parts, so I should feel no guilt).
Furthermore, there's likely some area for improvement in ride quality by optimizing the topout springs but I'm not going to fall down that rabbit hole.