When taking too much stress, one can take consequences. However, can one - at the same time - have both a mental and physical consequence of the same level or is there only a single 'pool' of consequences?
You can fill your normal (free) Consequence slots with either consequence type, but only one Consequence per slot. If if one of those is filled with, say, Deep Cuts, you can't also fill it with Spooked or the like, as it is already filled.
However, some skills give you special, extra slots. Those can only be used to absorb Consequences of the appropriate type (not just any type). The fifth level of Conviction grants you an extra Mild Consequence slot that can only be used to absorb a Mental Consequence (incidentally, it also gives you mental-only Stress Boxes at first and third level); the fifth level of Endurance works the same for Physical ones. But you can have multiple types of these slots if you achieve high levels in multiple slot-granting skills.
So if you somehow end up having fifth levels of both Endurance and Conviction, you now have: your 'universal', free Severe, Moderate and Mild boxes (of which each can be used for one consequence of any time when you need it), a bonus Physical Mild box that can only be used for physical ones, and a bonus Mental Mild box that can only be used for mental ones.