You want to break the jamb nut loose first, don't try to turn the adjuster with the jamb nut tight it will break off the adjuster head.
Dont worry about it if they both turn, maybe make a mark on the adjuster head with a marker so when it turns you know how much. Once the jamb nut is loose then try to hold the adjuster while backing off the jamb nut. Maybe use a metal brush to get some of the rust off before doing this, good thing you used some penetrating oil first it should help.
Or just skip this step and try to get the servo out. It can be a pain to get out also, just keep working at it.