Hmmm, I thought we were talking about DDRT - my bad.
For SRT, either a Frog sit-stand or a Yoyo, I do attach a tether from the foot ascender (upper ascender) to my harness. Why? Why not? It only takes about 10 seconds; I keep it attached to my upper ascender in the gear bag, so all I have to do is clip the free end into my harness.
I use a 6 foot length of Velocity rope as the tether, with a microscender to adjust the length. When I stop climbing, I pull the tail of the rope through the microscender to take up slack so if the main ascender slipped, the upper would catch me. Is that likely to happen? I doubt it; then again has an ascender ever failed? If it has, then it could, and it could perhaps happen to me. If it does I'll have a backup that took me all of 10 seconds to hook up.
But, if it isn't important to have a backup to the main harness ascender, then why is it important to backup a single ascender attachment point on a rope during a changeover? Why not just rely on the security of the single ascender in a change over just the same as we do during climbing?