Rob Walker provided the lower voice saying guilty.

ElRon recorded multiple lead guitar tracks in 2013 for Melody’s My Famous Last Words. Since Scomber used that backing track to write the lyrics and melody for Guilty, made sense that one or more of ElRon’s guitar tracks would fit.

East West Hollywood Backup Singers plugin was used on the scat line in unison with Scomber. Plus a second Scomber voice pitched to a -5th was added at a low volume.