That's a valid point re: a second guitarist although Zep RARELY ever used other musicians onstage. I believe that Atlantic Records VP Phil Carson played bass once, Richard Cole played tabla drums on "Whole Lotta Love" once and Keith Moon famously played drums once live at the LA Forum on 06/23/1977. But those were basically one-off performances and I don't think that Zep were very interested in using other musicians, and that of course made Page's job challenging for the need to be selective in his playing. Zep surprisingly pulled off "Ten Years Gone" in 1977 with Jonesy playing bass pedals and 12-string guitar together.
Also, re: "Sick Again", by 1977 Jimmy was performing the song coming out of "The Song Remains The Same" using the double-neck guitar's 6-string neck rather than the Les Paul, and I don't know if that made any difference in his sound or playing. https://www.youtube.com/watch?v=1iFwqXSQn20 Peace.