twocicle

Yes, the BH90 is now used for the current the road (ultegra & dura ace) and mtb (xtr, xt, saint) hydro brakes. Jagwire has a "Pro" hose kit that works ok on the mtb rear hose run, but it is still a bit more mushy than the BH90. With the road levers, the best firmness with decent modulation is to use BH90.

The 2 piston r785 calipers work fine on our road tandem at a loaded team weight of maybe 280lbs. For our mtb tandem where we ride quite steep technical terrain, definitely that needs the 4 piston M8120 calipers along with the H03C metallic pads which don't fade or burn out as fast as the resin/organic pads. I would definitely use the 4 piston calipers for heavy use.

The big "gotcha" with the rear calipers is when they require a banjo fitting. Shimano only has this fitting factory installed on hoses up to maybe 2000mm which is just a touch too short for our tandems. The Jagwire Pro kit mentioned above does have a banjo fitting which is one reason why we use it on our mtb tandem. A new solution to this issue is to get a hose coupler to join a standard rear Shimano BH90 hose (w/banjo factory installed) to a supplemental length of hose running to your levers. Otherwise, more recently there are a few after-market banjo fittings becoming available, mostly from China and posted on eBay. Amazon is also showing some postings but limited availability.

Here is a view of the coupler :