How to mate two pressure-treated 2x6s together?
I usually nail them together. Construction adhesive won't be enough. You could use screws also. 

For spacing I usually start by making sure the crown is going the same direction and at one end put 2 in within a few inches top and bottom, then at 6" away from those put another nail on the top an inch from edge another 6" one an inch from the bottom another 6" another at top and repeat until close to the end of the board 2 like the start. 

If your lumber has different bow in each board this can be pulled to match as you nail make sure to check that they still match at the loose end after each nail and it is usually easy to keep them even.