This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
To record the video and HiFi sound signals onto the same tape
area, VHS HiFi uses "depth multiplexing", while Beta HiFi uses
"frequency multiplexing". That is, the FM signal for Beta HiFi
occupies a different frequency band than do the Beta format's
luminance and chroma signals, and is simply mixed with those
signals and laid down on the tape by the video heads. In VHS
the luminance and chroma signals were too close together in
frequency for this to work. VHS HiFi uses a separate pair of
heads on the spinning head drum to record the HiFi carrier.
These heads' gaps are shaped so that the HiFi carrier is
actually recorded at a different depth in the tape than the
luminance and chroma signals.