This admittedly distant recollection, related by Johnston to Sony producer Steve Berkowitz, is reported in Michael Fremer's 2013 AnalogPlanet review of the Mobile Fidelity Blonde On Blonde stereo vinyl reissue:
"According to Dylan reissue producer Steve
Berkowitz, who oversaw this Mo-Fi reissue and who visited here a few days before
this reissue arrived, Bob Johnston told him that in Los Angeles (not Nashville)
he and Dylan spent three or four days on the mono mix. Dylan split and Johnston
then spent about four hours doing the stereo mix."
Elsewhere, Berkowitz recalled Johnston saying in
1999, "We mixed that mono probably for three or four days, then I said ‘Oh shit,
man, we gotta do stereo.’ So me and a coupla guys put our hands on the board, we
mixed that son of a bitch in about four hours!"
See this interview with
Steve Berkowitz and Mark Wilder. It seems unlikely that Dylan would actually have sat in with
Johnston the whole time, but he evidently had significant input to the changes
made. Both Robbie Robertson and Al Kooper were also
reportedly on hand, and may also have had some influence. Kooper's presence is
mentioned in Daryl Sanders' book That Thin Wild Mercury Sound (p.178),
and Robertson's in his autobiography, Testimony (p.222). See
Appendix F for detailed references.