not sure what they are telling you.
I have done this mod on 2 different cars. I know several people on this board have done it.

lets be sure we are clear.
you have the old single field voltage regulator.
there are 2 wires going to your alternator.
1 to the big stud on the back and the other from the vr to the field connection on the alt.

you want to put in a dual field alt but retain the old single field vr.

As stated above, plug in the single field from your vr to either of the field connections on the alt and then ground the other field connection on the alt.

I do not know what 2 wires they are telling you to splice, but it doesn't make any sense.
if you look at the wiring diagram for a dual field alt vr, you will see 1 field shows full 12 volts to the alt. The other is driven from the vr.
I have no idea what it does if both are driven off the alt, if that is what they are telling you to do.