After a little playtesting:
When performing multiple actions, the total bonus from skills on all actions taken in the round must be at least one step lower than your highest skill.
You may voluntarily reduce a skill in order to use it (so if you have Great Lore, you may choose to instead roll it at Good).
You may only apply a bonus from a stunt or power (including Focus Items, Weapon Ratings, and other similar effects) to a single action. Fate points or aspects invoked to improve a result only apply to one action each (so you can boost two actions, but that requires that you spend two FP).
So if your skill cap is Superb, you may roll:
One Superb
or
One Great
or
One Good and One Average
or
Two Fair
or
One Fair and Two Average
or
Four Average
Roll once, adding the bonus or penalty from the dice to all actions taken. You may re-roll by spending only a single Fate Point.
Notes:
-Limiting it by only by Skill Cap meant that if you wanted to use any skill lower than your top tier or "peak skill", you'd always opt to also take a second action since there was no penalty for doing so. This was also strictly better than taking a supplemental action in every way.
-This encourages the use of skills below your top two tiers. It's my experience that most players don't look below these in a conflict. This has the effect of rewarding the use of these by in essence granting an extra action.
-This essentially imposes a -2 penalty on the main action if you wish to use an extra action. Since most "supplemental actions" are 0-1 shift effects, the greater penalty allows for potentially better results than just taking a supplemental action. Other penalties are built in as well (in that there's functionally no difference between rolling a Fair skill at -1 and rolling an Average skill).
-Limiting bonuses from anything other than skills was necessary to prevent abuse. Same with FP, obviously.
-I found that using a FP to re-roll the dice, however, is not significantly better this way due to the probability differences.
-Having a single roll speeds up gameplay considerably, but doesn't generally change the effectiveness of this option.
-As much as I really, really want to, having stunts affect the cap on this is a very bad idea. It's basically giving free actions and leads to a dramatic increase in power that is also difficult to measure due to its versatility.
-Taking only one Superb action is generally better than taking two actions (one at Good, one at Average). Taking a Superb and a supplemental is about the same as taking one at Good, one at Average (or two at Fair). Taking one skill at Great is slightly suboptimal in this scenario (but not significantly so). Taking lots of Average actions is generally suboptimal.
-This can complicate and lengthen conflicts considerably. Be warned.
-I did limited testing with a cap of Great and Good. I did no testing with a skill cap higher than superb.
-Testing was done both with the DFRPG assumption that 3 shifts is enough for a successful, unopposed maneuver and the Fate Core assumption of 2 shifts. This is obviously better (read: more advantageous to the player) with the latter.
-If using Fate core style results (boosts and succeed with style), rolling one Superb skill (unopposed, difficulty Fair) will most often result in one Aspect and one Boost. Two Fair skills will only result in two Boosts.
-If using DFRPG "sticky" aspects (with Good difficulty), there is rarely an advantage to taking multiple actions which aren't movement related.
Take these as you will.