Does Wasteland Return speed stack? and if so how?

In Fallout Shelter you can get pets that increase the return speed by cutting the timer.
For example, if you got a pet with Return-speed X4 a trip of 4 hours becomes one.

Now seeing you can have up to 3 pets accompanying your team, does the return speed stack?
And if so what would the calculation be?

For example if you got two Return-speed x4 pets will it be:

4/4 = 1 / 4 = 15 minutes

OR

4/8 = 30 minutes.