Using abilities together (specifically, outside of BDtP)

A player of mine and I began debating the finer points of using two abilities together during a complex conflict resolution, and I would like some opinions from you fine folks.

The other day, one of our players said that she wanted to take out two bandits that she was walking with.  We let her do a Stealth roll and add the success outcome to the second roll, her Spear-Fighting.  It was great and seemed to come off perfectly.

Another player has discussed using Deceit and Sway together.  Specifically in a scenario in which he was trying to A) convince a Joe the Guard that he was also a guard, instead of having just knifed Frank the guard and stolen his uniform.  and B) that Joe the Guard should let him pass into the restricted access chamber beyond, which Joe is guarding.

My concern stems from that this initially felt like there was the possibility of overlap between the rolls.  Deceit and Sway accomplish the same end result, just by using different means, was my argument.  This feels like its somewhat an excercise in symantecs, as we debated the differences in stealth vs. spear-fighting, and deceit vs. sway.

Before this turns into a 'Long Story'... let me try to make this a simple question:

How do you (or your players, as a democracy) go about deciding what makes good/valid ability combos? 
Maybe I've over-considered this, and the answer is "Does it sound fun and exciting?  Great! Go with it!"

Thanks!

Let's get some inspiration from the rules:

The wrote:
Using abilities together
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If you want your character to perform a complex action that uses two abilities together, decide with the Story Guide which ability is most appropriate to the action and which is secondary. The secondary ability is used first and the success levels are used as bonus dice on the second ability check. The Story Guide and player will have to decide what happens if the first ability check is failed: in some situations, the second ability check can still be attempted without harm; in others, the ability check can be attempted with a penalty die; and in others still, the second ability check cannot be attempted.

A character is trying to cut a thong from a guard's belt and snatch his keys, using Bladework to chop the thong, and Stealth to grab the keys without being seen. While she is using Bladework to actually get the keys free, the Stealth part of the action is most important. The player makes a Bladework ability check. If successful, the success levels are converted to bonus dice on the Stealth ability check. If unsuccessful, however, the keys are still on the guard's belt, so the Stealth ability check cannot be attempted.

Another character wants to approach a wild bear without getting attacked using Animal Ken. In order to help with this, she's going to attempt to remember what bears like to eat and see if she can find some, using Woodscraft. The Woodscraft ability check is secondary, and if successful, will add bonus dice to the Animal Ken ability check. If unsuccessful, there is no complication; the character just must approach the bear with no food gift.