You'd be challenged to pick two beers so far apart on the carbonation levels for their styles.
+/- 0.5 volumes, the porter wants to be down in the 2 volumes neighborhood, while the hefe usually runs around 4 volumes. Referring to
this carbonation chart, to achieve those levels at 40°F would require 7 psi and 29 psi, respectively. That's a 22 psi differential.
I picked that temperature because it's the highest that 4 volumes still fits in the chart. And even while there is no data for 70°F, if you look at the 2 volume vs 4 volume curves, the differential only gets larger with increased temperature.
So I'd say there's no
practical sure-fire way to deal with that with a single regulator. For an impractical and/or hit-or-miss way, set the CO2 up to carb the hefe to 4 volumes, and take samples of the porter day to day, and take that keg off the gas once it hits a nice carb level.
Otoh, if you could live with both brews in the middle, say around 3 volumes, you could dial that in pretty easily, just use the chart and extrapolate a little bit...
Cheers!