The deploy mines activity can be performed with one or two actions, however it looks like there are a number of class feats that are only balanced, power wise, if deploy actions were a 1 action activity.
The following are some of the feats that may not be properly balanced if deploy mines were executed as the 2 action version:
Instant deployment
Mobile deployment
Double deployment
For example, double deployment and mobile deployment both would let you compress what would otherwise be 4 actions into 2, saving 2 actions, but most all similar class feats from other classes only save 1 action, and when they do save 2 actions, they usually have a restriction attached to them.
Also note that once you start combining these feats with some of the other mechanic class feats, you can do a very large amount of damage even at low level. Consider double deployment combined with proximity alert mines or double detonation. With double detonation, I can do class level+bonus damage by using double deployment as 1 action, then double detonation as another action, and I have an action left. With proximity alert mines I can do the same thing with 1 action, and since there is no limit on how many mines detonate a round, I could use 3 actions and 6 mines to deal 3xclass level + bonuses damage to everything in the area. So, 30d6 + bonuses damage in an area at 10th level (costs 6 mines).
I don't know if this would entirely fix the problem, but I'd suggest something like the following:
Instant deployment and mobile deployment both specify the one action version of deploy mines.
Double deployment takes 3 actions and specifies that you can use the 2 action version of deploy mines. This makes sure that you can't use proximity alert mines with it more than once a turn.