Well my comment would be it will probly work fine, but I would adjust them so they produce the same spl..If one box is louder than the other the the difference in pressure can "steer" the sound wave. Now this might be something you want or need, and then it might make things worse depending on your setup. Although I have never used those particular subs i have used some of the LABhorns.. But my personal preferance is to use the same subs all the way across, unless there is some good reason not to.
I would not run them at the same SPL level. If you do, the TH118 would be nowhere near its maximum output at the point the TH115 would be at its' maximum. So in essence the TH115 would be "working harder". And the maximum output of the whole rig would be less (limited by the TH115).
If they were run at the same "potential", then they would both be working less hard-for the same SPL from the rig-as if they were run at the same SPL.
Unless you are running a large number of them in some unconventional manner, there will be very little steering. As a general rule "steering" is done by adjusting signal delays. Just because one cabinet is louder than another does not mean the sound is "steered"-except that it is louder in front of that cabinet.
The Tapped horn is a very different design than the Lab sub-which is a conventional bass horn-albeit it with a different approach to the drivers. But that has nothing to do with the cabinets could be run together.
The phase response between the 2 cabinets is identical-so mixing and matching is not going to be an issue.
Where you run into problems is using subs that have a different phase response-and the cancellations that occur because of that.