You can always add a single button satellite shifter on the base bar. So you’ll have the single button hydro shifter/brake plus a single button satellite/sprint shifter.
Or you get a mechanical/hydro brake caliper, like the TRP HYRD or Juin F1 brake to pair with an old di2 2 button brake lever. Alex Arman
That would overcomplicate things, since the TT ultegra di2 lever does not have a port for the satellite shifter. Yes it could be solved, with redundant junctions etc...
Also, I was looking at hydraulic. Mechanical disc is suboptimal IMO Sr. Salitre
Only other option I can think of is to hack together a non hydraulic 2 button onto a hydraulic lever! Can be done, but a bit of work! Ask me how much I love my Kiwami LD Aero Trisuit
...which is what I wanted to avoid in first place. The option to disable/override synchro shift is a must for me.
You can still get the two button bar end shifters. If you use those then you can still use manual mode, you just won’t be able to shift the FD from the base bar, but that’s not a big deal.