i can tell you that that won't be that much fun. for what little money a second bit ($300 plus or min.) will cost you i'd just get a second one.
My 15 minute/post included everything to get a post in the ground, e.g., positioning the tractor at the spot you want the post, leveling and positioning the post. The actual driving of the post once this is done is only minutes. Working alone increase the time as well. Two people saves a lot of time getting on and off the tractor when positioning the driver. Even so, it took me about 2 months of evening and weekend work to drive both wooden and metal T posts.
Once it was located, I dropped the 3ph to set the driver on the ground. Then I used the 2 cranks (one for left/right, one for fore/aft) on the driver to get things vertical. Though this description sounds lengthy, even working alone I average 15 minutes per post. A far cry from how long it would have taken using an auger. Of the 250 wooden posts I drove, I think I had one split, but that was caused by a knot near the top of the post. I had one or two T-Posts start to bend when they hit a large rock which caused me to have to relocate the post. Some of the wooden posts are not perfectly vertical because they go off track when they encounter a rock, but because I was using electric tape and not boards, this was not an issue.
My post driver came with an accessory for driving T-posts. It looks just like the manual T-post driver (a pipe that's closed on one end with handles on each side), except instead of handles it has 2 plates set at 90 degrees to each other that allow it to fit securely in the channel. The ram drops down on this instead of directly on the T-post. Hard to describe, but it worked.
