Good morning Dave and happy new year, glad I was some help. The T-K1 is the ultimate tankless heater (except for it's big brother,the 235,000btu T-M1 Mobius, which modulates down to 25,000btu) and will definitely supply two simultaneous showers, I guarantee it. The TK has a built-in automatic power vent, and uses 4" steel vent tube. The 8-9 ft distance to your outside wall, is no problem whatsoever. The TK blower can handle up to 21 ft horizontally! We also supply the heater mounting brackets and vent terminator, for outside the wall. The TK also has full electronic ignition and burner modulation for precise temp control.
If LOTS of hot water is needed, think about the T-M1 "Mobius". One heater can produce almost 10 gpm. Designed as a commercial water heater, as many as 20 of these may be manifolded and managed by one programmable controller. 20 heaters can produce 200 gpm or 12,000gph, it blows the mind doesn't it.
Tankless has come a long way.
I'll e-mail you some info, Dave. Thanks for asking.