That is how Airstream built many of the tandem axle trailers back then.
The theory was that with one axle hydraulic and one electric, it would be near impossible to have both fail at the same time, therefore one still would provide some braking power.
The fix is to put new backing plates on. But then you will have to add "armature plates" to the hydraulic axle hub and drum assembly.
Four backing plates would be required since the new style brakes are not compatible with the electric brakes you now have.
The down side to all of it, would be that you still would have axles with small spindels, that have been known to frequently fail after many miles, along with two piece hub and drums that are normally "way out" of balance. And finally, the axles themselves may be bad.