Completely move high frequencies from stereo to back channels in wav files

Question

I use bi-amp'ing and have idea convert stereo wav files to 4 channels - front channels with low frequencies and back channels with high frequencies.

Can and how I do modify wav stereo, for example on 3,000 Hz, to 4 channels wav?

2022-02-06:

Microsoft Windows [Version 10.0.19044.1466]
(c) Microsoft Corporation. All rights reserved.

C:\Users\Public\Downloads\ffmpeg\bin>ffmpeg -i in.wav -af "lowpass=f=3200" out-.wav

C:\Users\Public\Downloads\ffmpeg\bin>ffmpeg -i in.wav -af "highpass=f=3200" out+.wav

Strange, output file size equal to input file size,

And one more option,

C:\Users\Public\Downloads\ffmpeg\bin>ffmpeg -i in.wav -filter_complex 'acrossover=split=1500:order=8th[LOW][HIGH]' -map '[LOW]' low.wav -map '[HIGH]' high.wav
ffmpeg version n5.0-4-g911d7f167c-20220206 Copyright (c) 2000-2022 the FFmpeg developers
... skiped ...
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, wav, from 'in.wav':
  Duration: 00:01:42.23, bitrate: 1411 kb/s
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, stereo, s16, 1411 kb/s
[AVFilterGraph @ 000001c5bc017300] No such filter: 
'acrossover=split=1500:order=8th[LOW][HIGH]'
Error initializing complex filters.
Invalid argument

What difference?

SOLUTION

HZ="2700"
sox inp.wav "$HZ"-.wav lowpass $HZ
sox inp.wav "$HZ"+.wav highpass $HZ
sox -M "$HZ"-.wav "$HZ"+.wav out.wav