The axle mostly needs to be restrained from twisting. What you have there is a lot like a coaster brake bike without a torque arm.
File out a heavy steel tab for each side which closely conforms to the flats on the axle, and have it attach to the fork like a coaster brake reaction arm.
I believe that what is happening is the flats cam themselves out of the dropout by twisting.
I like the Burkhardt tabs but feel it would be good enough to attach them to the fork legs with a strap and let the axle nut handle everything but the twist.
BTW if you did splice two forks together I think you'd be much better off building up the steer tube of the chrome fork and leaving the crown uncut and unwelded.