In message , Old Fart at Play
writes

Chris, you are still confused but nearly there.

As you say, a 4th order L-R is two 2nd order Butterworths.
At the crossover frequency a B2 filter is -3dB and 90 degrees phase
shift.
Therefore the 4LR is -6dB and 180 degrees.
The LP and HP outputs are in phase at all frequencies
and the voltage sum is constant.

HTH,

Roger.

You are quite correct, I'll go and hang my head in shame now :-(

For a 4-th order L-R, you do indeed cross over at the -6dB point and the
outputs of the LP and HP sections are always in phase. (And the
magnitude sums to be flat).

(I even went and SPICEd it to check!)
--
Chris Morriss