No, you just group turrets together, and if the first in the list fires first, it will activate the combo. In my Tazer+Rocket Launcher combo, the Tazer's range is not enough for it to hit anything, so that it will always combo with the Rocket Laucher. The best way to experiment is with two blue turrets, because there is no way for that to work incorrectly. In the end (like I said earlier) the only thing that kills them is the Black Hole.
BTW, all of the turrets in the combo need to have maxed damage.
http://onslaught.playr.co.uk/about.combos.html
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