Replacing the T&B by a second approved attitude indicator is allowed as an "FAA-approved equivalent" in Advisory Circular AC-91-75. This is not a new regulation since the reg (91.205) allows for "FAA-approved equivalents." This interpretation only applies to Part 23 airplanes under 12500 lb. A power source separate from the primary attitude reference is required.
The T&B is normally required as a relatively-cheap back-up to the attitude indicator. Because of its construction it is more rugged than an attitude indicator as is not succeptable to gimbal lock or tumbling. In a slow, straight-wing airplane it lends itself to instrument flight. In a fast airplane, it is far too insensitive.
The turn-coordinator is a T&B with the gyro axis tilted so that it is responsive to both yaw rate and a little bit of roll rate, making partial panel turns a little easier. I understand from some old-timers that the moving airplane format was chosen so that pilots could tell the turn-coordinator from the attitude indicator.
Back in the old days, my instrument IP (no it wasn't Orville Wright) didn't believe on those new-fangled attitude indicators and I hardly had a chance to use one until I had my instrument rating.
Goldfish