Fast Completion Skill Challenge
Q: Can skill challenges reduce the time of an action to a free action? There were worries that this might lead to "always on" abuse for some skill challenges.
If the skill check could normally be made as a move action, yes. See page 77. GMs should remember and enforce their right to limit the number of free actions allowed in a round.
I know this design was intentional because I argued against it during playtesting. My personal house rule on this is that you can use a skill challenge to reduce a skill check to a swift action, not a free action.
Swift Action: A swift action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action. You can perform one swift action per turn without affecting your ability to perform other actions. In that regard, a swift action is like a free action. However, you can perform only a single swift action per turn, regardless of what other actions you take. You can take a swift action any time you would normally be allowed to take a free action.
Note also that skills where a character moves as part of the action (e.g. tumble, jump, balance, climb) generally cannot be performed as a free action (though they can sometimes be a reaction, e.g. being hit while climbing).
Simultaneous Action Skill Challenge
Simultaneous action challenges are for just that: simultaneous actions. If you're trying to accomplish 2 different things in the same action (free, move, standard, full round), the 2nd thing has to be done with this challenge. This is one of the reasons the fast completion challenge is so powerful, and why I recommend allowing actions to be reduced only to a swift action, not a free action.
Note that the Iron Heroes book specifically notes that this challenge only applies to two things you couldn't normally do together. What constitutes "could(n't) normally do together" is left to the GM, so it's a bit of a judgment call.
If two different skills are being checked, I'd generally expect that to be a case where this challenge applied (exceptions would be rare).
If the same skill is being checked for two different purposes (eg break fall and tumbling mobility on the tumble check) then it depends exactly what those purposes are. If one or both purposes have an effect on another character (denying them Active Defense or an Attack of Opportunity for instance) or on the performing character's attacks (such as leaping strike), then I'd definitely require the challenge.
Note that the simplest form of application is to say "two skills or one skill for two purposes, it's always a simultaneous action challenge". There's fewer arguments this way (though slightly less cinematic action).