The combined skill rolls mechanic doesn't really make sense to me as written. On page 407 of the Keeper's Rulebook it states:
Used when a situation allows or demands the use of more than one skill.
Only one dice roll is made; the result is then compared with each of the skills named. The Keeper will specify whether a success is required for both skills, or if only one of the skills need be successful.
If the keeper requires success for both skills then that is completely equivalent to requiring only success for the skill with the lowest chance since success is then automatic for the skill with the higher chance so really only one skill matters. If instead he requires that only one skill is required and specifies the skill then again only that one skill matters. If he requires that only one skill needs to be successful but doesn't specify which one then only the skill with the highest chance matters. In none of these does the outcome depend on more than one skill and the reference to a combined skill roll doesn't matter.
Why not just say that if there are multiple skills that apply to a given situation then the Keeper can choose either to require the investigator succeed with the skill with the lowest chance or if he sees fit, with the skill with the highest chance. In all such cases only one dice roll will be used to determine the outcome.
On page 203, it states:
Which skill to use for perception can be open to question, depending on the situation. If a non-player character is about to draw a weapon to attack an investigator, the player might make a Spot Hidden roll to see the weapon being drawn or a Psychology roll to become aware of the non-player character's intention. In this case allow the player to make one dice roll using the higher of the investigator's two skills (see Combined skill rolls).
If it's just using the skill with the higher chance is this really a "combined skill roll"?
On page 93, we have:
Later, Harvey attempts to repair an electric turbine. The item is both mechanical and electrical, so the Keeper asks for a combined Mechanical Repair and Electrical Repair roll. One roll is made and the result is compared to both skills—in this case both must be successful to achieve the task.
Or rather, he must succeed at the skill with the lowest chance, since then he automatically succeeds at the skill with the highest chance.
In the example on page 204, It states:
Harvey is a guest of Lord Foxton, who has chosen to drug Harvey's coffee. The Keeper wants to give Harvey a chance to notice this and offers a perception roll. The Keeper allows Harvey's player to make a combined dice roll, comparing her rolled result to both Spot hidden and Psychology. If the roll is made using Psychology, Harvey would pick up Lord Foxton's anxiety and sense that something is wrong. If it were made using Spot Hidden, Harvey would notice that his coffee smells a little strange.
It's not clear why this should be a combined roll since the two things being rolled for are completely separate. One is the demeanour, etc. of Lord Foxton and the other is the odour of a cup of coffee he is holding in his hand and possibly drinking from. If he succeeds at the skill with the lowest chance does he then get both pieces of information? If not then which piece of information does he get.
In my view the combined skill roll mechanic should only be applied to situations where multiple skills may apply to a single distinct outcome. This is the case, for example, with the examples quoted from page 204 and page 93. In the first the result of succeeding is that the investigator becomes aware he is about to be attacked and in the second he succeeds at repairing something that requires both skills. In the example from page 204, if he succeeds at Psychology he becomes aware that Foxton is anxious, worried, annoyed, insane or whatever, while if he succeeds at Spot Hidden he becomes aware that his coffee smells strange. It would make more sense to make the Psychology roll first and if that succeeds to then roll at a reduced level of difficulty to see if Harvey detects the odour from his coffee (since coffee could be quite good at masking such smells).
Thinking about it further, in the case of the repair requiring electrical and mechanical skills, what if there is another investigator present, with say, a better electrical repair skill than Harvey. In that case I'd say the rolls really should be separate since Harvey may succeed at the mechanical repair while the other investigator may succeed at the electrical repair.