Creatures that enter the battlefield due to a single effect or ability enter the battlefield at the same time (and, unrelated to the question, see each other). That means Containment Priest isn't on the battlefield at the time either of the two creatures enter, meaning none of the two would be exiled.
If by any chance you mean someone puts a creature on the battlefield that wasn't cast, and a player puts Containment Priest on the battlefield in response without casting it (for example with AEther Vial), then the second creature would be exiled because at the time it enters, the Priest is on the battlefield already.
December 5, 2014 1:02 p.m.
Containment Priest has a static ability that defines a replacement effect. If it were a triggered ability that triggered on creature ETB, Containment Priest would exile both creatures in this scenario.
In order for a replacement effect to apply to an event, it needs to exist before that event would occur. Containment Priest's replacement effect doesn't exist until Containment Priest is a permanent, so it can't apply its own replacement effect to itself or to any other creature that enters the battlefield simultaneously with it.
December 5, 2014 1:07 p.m.
I was thinking of an effect like Planar Guide. So it wouldn't work in this instance