Which Way Does the Pet Door Control?
The smaller size electronic and magnetic pet doors for cats to small dogs usually control in one direction only. A small peg is raised on the inside of the flap and that peg does not release unless the signal from the collar key is present.
However, the flap can still be pushed toward the outside (unless the 4-way lock is set for 'in only'). For this reason, these are not suitable for allowing one pet out while keeping another in (more below).
You might suppose that one way to use the one-direction locking pet doors to keep one pet in while allowing another out would be to reverse the pet door so that it controlled pets on the inside but not, obviously then, on the outside.
That idea works provided that you carefully shield the now outside portion of the pet door from the weather. Also, be aware that some warranties are voided by this type of installation.
Another idea that we have encountered is installing two pet doors--one at each end of a fabricated wooden tunnel made long enough that the flaps don't interfere with each other. Then the outside unit controls access from the outside and the inside unit controls access from the inside. Such an installation would be more suitable for a wall than a door.
Some pet doors control in both directions. We think these will be considerably better at resisting wild animal attacks--particularly raccoons.
Doors that lock both ways will also allow an animal to go out while keeping another in so we think this is the best choice for the very common situation where the dog is allowed out but not the cat.