Game Text:

Watch My Garden Grow: At the beginning of your turn, you may generate a hindering terrain marker in a square within range.

My Plants Have a Calming Effect... Or The Opposite If I Desire: Smoke Cloud. FREE: Make a close attack targeting an opposing character occupying a hindering marker generated by Gardener regardless of adjacency. If you hit, choose that a hit character either "can use" or "can't use" Battle Fury until your next turn.

Question:

If two Gardeners have placed their hindering marker via "Watch My Garden Grow" in the same square can they both make attacks?

No. Only the top most terrain marker would be "active" for any effects.

Can you direct me to the applicable rules text to this affect? Thank you.

Rulebook on "Markers"

If multiple terrain markers are in a square, the most
recently placed terrain marker (the one on top) determines
what type of terrain it is. The others exist below it but
don’t affect the terrain of that square or have any other
special effect, except for an effect that would remove them
from the map. If the top marker is removed, the square
immediately becomes the terrain of the marker (or printed
terrain) below that.

Core Rule Book p 22

Ninja'd

To note, if you wanted to do that, you still can... SC with gardener 1, attack with him, SC with gardener 2, attack with him. Aside from mechanics like this, there's no reason to put 2 smoke clouds on top of each other and that's the only viable way to get both of their effects to work.

begining of turn effects must be complete before any actions are declared.. (even free actions).

so, #1 & #2 place their hindering on different squares.. no reason to place them on the same square, since only the top one would be active.

#1 does the free action to attack.
#2 uses smoke cloud to also include the target of #1.. replacing his terrain marker..
#2 does the free action to attack.
.. swap the costed action every turn.

might work even better if you can give #1 knockback on its attack.. that way it can push it onto the hindering terrain that #2 placed. Then you don't need to used smoke cloud.