I've been using, and have thoroughly pressure tested, the use of 7 gr 4759 under 52 gr of Goex Cartridge, FFg and FFFg using both the Lee 459-405-HB (410 gr bullets +/-) and the Rapine 460500 (500 gr bullets). It is my standard load with either bullet using any of those powders. Velocities stay consistent at 1350 fps +/- and 1250 fps +/- when replicating the M1873 or m1882 ammunition.
The SAMMI MAP for the 45/70 Government cartridge out of deference to the M1873 Springfield Rifle is 28,000 psi [transducer or strain gauge] . The 7 gr/52 gr BP load runs close to the same or about 1,000 psi higher than the psi of the same powder loaded at straight 70 gr. The measured psi runs from 17,000 to 23,000 psi [depending on bullet and granulation of the BP] with any of those Goex powders.
The highest measured psi I've recorded with BP in the 45/70 Government cartridge with straight 70 gr load or 7 gr 4759 under 54 gr (2 gr more than the OP is using of BP) was with some original Dupont FFFg "Superfine". It has the granulation of current Goex FFFFg BP. the measured psi ran 24,600 psi with a straight 70 gr load and 25,000 psi with the duplex 7/54 gr gr 5759/Superfine load. Both still well under the SAAMI MAP for the cartridge for use in the M1873 Springfield Rifle.
During the same testing session I pressure/velocity tested some original factory loaded REM-UMC 45-70 Govt cartridges. They had swaged 405 gr bullets (lube grooves were quite shallow and were canalure rolled into the bullets. The 70 gr powder charge was heavily compressed and the granulation was close to that of Goex 4f and 3f. The measured psi was 22,000 psi and the velocity was 1233 fps which is at the top end of any measured psi's of the duplex load [7/52 gr].
The idea of using these kinds BP loads is well explained in Spence Wolf's book. The main purpose is to blow the fouling out of the barrel [the OP has discovered the recommended duplex load does indeed reduce the BP fouling left in the barre thereby negating the need for "wiping" or the use of a "blow tube" and accuracy is maintained over many more rounds .....the use of this type of duplex load is not to increase the velocity or pressure over straight BP loads. This type of "duplexing" reduces the amount of PB used an appropriate amount for the smokeless powder used thereby maintaining the intended velocity and pressure. It does indeed accomplish that.
Concealment is not cover.........