To my knowledge, there is no such animal, though you are right, it ought to be doable in a digital gauge. I bet if somebody sold one it would be real popular.
I looked into this quite a bit in planning my gauge layout and the closest thing I could find was the ever-popular Scanmaster. It displays battery volts, coolant temp, rpm, mph, intake air temp, plus all the usual SM tuning parameters (02 volts, BLM, knock retard, etc.). And I'm probably leaving some stuff out. I got mine for $200 used and mounted it in the dash above the radio. A pretty good deal for all that info. On top of this I added separate VDO gauges for oil psi and coolant temp (mounted in the center console); and rpm and boost (mounted on the column).
Other options:
--Some turbo timers also display boost and volts.
--Westech makes some dual readout mechanical gauges.
--There's a high-end racing gauge maker called SPA that sells really nice dual-readout digital gauges, some programmable with built-in warning lights, but they're pricey (~$200/gauge). Available from
www.racerpartswholesale.com