I believe the rear horizontal hold control is basically a coarse frequency adjuster and the front one is a fine adjuster. If both are working, you would set the rear one so that you have good horizontal stability with the front control set at the center of its travel. After that, minor adjustments can be made with the front control alone. In other words, the rear (coarse) control gets you in the right ballpark, and you make everyday adjustments with the front control. Your service manual should explain all of that, or at least give brief instructions for setting these controls.
If the rear control is a potentiometer, the simple repair is to replace it with one of the same value. While you're waiting for the new pot to arrive in the mail, I suppose you could experiment with substituting a fixed resistor for that control -- trying different likely values until you find one that gives you a stable picture. That's not a good long-term solution (there is a reason why the designers used a pot there instead of a fixed resistor), but at least you could watch the TV in the meantime.
If the rear control is an adjustable coil, maybe not so easy to find a sub. Moyer is sometimes able to supply vintage TV parts:
http://www.moyerelectronics.com/Depa...-TV-Parts.aspx
Just my $0.02.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html