After trying several times in co-op, calculating how much is being spent, we weren't able to build more than one Dark Temple of the Sun God. Has anyone been able to do this?
No it is not possible,
they were not kidding when the name of the Monkey Knowledge is There Can Be Only One.
Not due to the cash, as that could eventually be obtained in freeplay mode,
Not for the rule of being able to place only 1 of each type of Tier 5 tower, as this can be circumvented in co-op,
But because it is specifically designed that there can only be one in the game at a time.
Have I ever tested this? No, but there is someone who has with this youtube video.