Two high pressure injectors is too much added complexity over just firing with plugs. Most of the dual mode units I have seen online (never have seen a dual mode in person), are conversions that just had the diesel around already, so why rip it out.
Turbochargers and direct CNG injection with spark ignition is the way to go, IMO. Most diesel designs already take a turbo into account, and most modern ones even have wastegates that can be computer controlled (or already are).
I was jealous of a friend in the Silicon Valley area in california. He bought a well-used police CrownVic with CNG conversion. It needed a valve job on purchase (did not have the hard face valves that last better with CNG), but now he pays a pittance for fuel costs, and that area has plenty of CNG stations.
This isn't a "dual mode" unit. There is only one "mode" with with a tiny amount of diesel replacing spark ignition.
Note that I said for NEW rigs, which would come with the new fuel(s) cycle installed.
As to whether having two higher pressure injectors is "too much added complexity", I think that will depend on the efficiency and price differential.