</font><font color="blue" class="small">( It is a two bottom plow. I took one of the bottoms off it to plow with.
I would use the plow in the middle of the tractor for ditching.
For plowing I use the one near the rear tire. )</font>
Ah yes, that makes sense, for a one-pass operation. However, to get deeper, you want the right side tires to ride in the ditch on the second pass, so you'll want the plow that's in line with the tires to be the one you use to get deeper. It's likely you could easily go 14" or so this way. Our good friend, John Miller, (who seems to have retired from TBN) once posted a picture and short description of how many of us use a rear blade in the same fashion. The rear blade pushes the soil a little farther out of the ditch, but makes a tremendous rut/ditch after a couple of passes, and because of the nature of the shape of the blade, the slope is a little more gradual than a plow. Briefly, what you do is set the rear blade at a big angle, usually a 45 or more and then also tilt the blade low on the leading edge, so it ends up looking like a huge plow. If your blade doesn't have enough adjustment to to get the right position, you may need to pin/bolt it in some fashion to do this. What you strive for is to have the leading edge/point fairly well in line (directly behind) the rear tire. That way, with every pass, you just keep digging deeper, without ever even changing the blade position. I usually strive to take about 4 to 6 inches with each pass. Eventually, you get to the point where you are in a pretty big lean, and so it's time to quit. Only down-side with the blade method is you'll be piling the soil to the center of the tractor, so if you machine is very small, you may end up scraping.
I don't think you need to bury the tile all that deep to keep from crushing it. We've buried plenty of pipe with any where from 12" to only 5" of soil on top of it with no problem. Once the soil firms up, it's pretty safe to drive on. In fact, with all the clay around here, we've usually used the tractor, riding in the same trench on top of the pipe, to compact the soil afterwards.