Hey WDF--good to see you!
I really see no real advantage to using both MH and T5s. Generally the MH needs to be at least 6-12" above the canopy. And the T5s need to be within an inch or 2. It is quite hard to get the T5 close enough to do any good without blocking the light from the MH. If you have a small space and mount them on the walls, you will get foliage growing into them. This would work while the plants are very small, but when they are, you don't need that much light. When the plants get larger, I am quite certain that your entire space will be full and the T5s will just get in the way. I really see no way to use them effectively and not block the light from the HID or have the foliage grow into them and burn the leaves.
Did you buy a tent? How large is it? Do you need more light? If you have not purchased these yet, I wouldn't. I really think that you are going to find them more of a PITA than an asset. If you already have them, are they a single fixture with 2 tubes or 2 fixtures with single tubes? Are they HOs, how many lumens do they put out, what color? Let's make sure they are worth messing with. I think your only option is to mount them on the walls, rotate your plants, and try to keep foliage from growing into them. Overhead along side the HID they would be too far away to do any good. When you say lay them down close to the middle of the foliage, I assume you just mean hang them close to the center of the foliage with the HID overhead? If so, this would block HID lighting. Any chance we can get some pics?