Sounds like a bug to me. I never lost stability for accepting a peace. Anyone else?
Here's a tip on how to avoid taking stability hits from peace offers though.
I've played sweden a lot in various scenarios, and a lot of the time you're faced with some very rough two front wars. I know just the scenario you're describing, hehe.
A lot of the time, I just let Russia take a few provinces and suffer heavy attrition losses, while I finish off the danes before taking him on. A heavily fortified Ingermanland or Karelia during a hard winter will reduce his forces quite nicely
While fighting the danes, Russia will offer peace all the time, and once he starts taking serious attrition losses, or his economy is feeling the strain from the war, he will start offering some 'generous peace offers', that you will lose stability to if you decline. So, when I know that I can beat the computer later on (after I finish building troops, or after I finish a war on another front *cough* annexing the evil danes *cough*, etc.) I just let the peace offers (tiny icons in upper right hand corner) sit there and dont click on them. After a while they will go away and you won't lose any stability to declining them since you never even read them.
Once I get the upper hand in a war, I usually start reading them, since they never give 'generous peace offers' when the human player has the upper hand.