I was sure I heard a second guitar in there on ESL, and there are lots of pictures from that era with both of them on doublenecks, so I always assumed that that was the spot. But you're right; Google can't find me any pictures with both of them on six-strings at the same time. Geddy's doubleneck is a bass on top and a guitar below, right? I always figured that that's where he went to it.
So I listened to the end of Xanadu again, and it does sound more like Geddy is playing bass but some serious keyboards at the same time. Very big, very full sound behind Alex's wailing guitar, so I guess in my head there had to be a rhythm guitar in there. Geddy had a set of Taurus pedals but I thought this was before he had "real" keyboards on stage. To play chords like that, he'd have to have the pedals triggering something modular or programmable. The Oberheim would do the trick, but I didn't think he had that until later.
I haven't seen ESL the video in a while. I don't currently have a copy. 
If I'm not mistaken (and without doing any research), I believe Geddy had two doublenecks: one with a 4-string bass on top and a six-string guitar on the bottom (Rickenbacker 4080/6), which was primarily used for A Passage to Bangkok, and the other with a 4-string bass on top and a 12-string guitar on the bottom (Rickenbacker 4080/12), which was primarily used for Xanadu. The 4080/12 is black or dark blue and is the one he's most often pictured with. The 4080/6 is cream colored and is shown in this image (among a small handful of others):
https://1.bp.blogspot.com/-rDMsBKWTsy4/UietVssXtSI/AAAAAAAHfAU/IF8qza0HgJ4/s640/1170684_10151630216218479_654119533_n.jpg He used a sunburst model on the R40 tour, but I never saw that one in any earlier photos, so I assume it's a relatively recent addition to his collection. I believe he may also have used one of the 4080's for parts of Hemispheres, but that was before my time.
Here's the ESL version of Xanadu:
https://www.youtube.com/watch?v=JF9cpIlSdqo My statement that the ESL video never shows Geddy playing the guitar isn't completely accurate. Geddy plays the guitar (which I believe in this case is the 4080/12) during Alex's guitar solo (starting around the 10:48 mark on the video) and through the end of the song. The camera mostly stays on Alex during the solo. There is a cut to Geddy around 11:10, but it only shows him from above the top neck on up. However, it's pretty obvious from the position and motion of his arms that he is playing the bottom neck (he is also standing behind the keyboards, and his body motion indicates to me that he's playing the Taurus pedals (basically going back and forth from E to F#)). There's also a quick shot of Geddy strumming a chord at 11:41, but it's the same "above the necks" angle, so you don't actually see his hands. At 11:48, you can see him holding the bottom neck in his left hand while turning some knobs the keyboard with his right hand. At 11:50 he backs out of the frame to strum the guitar and them moves back into frame to hit a note on the keyboard. Geddy's guitar is most notable starting at 12:01 when he plays a descending single-note line while Alex is playing arpeggiated chords high on the neck of his 12-string. Finally, you can see Geddy strumming the song's final chord starting at around 12:03 (first in a close up shot from the side and then in a wide shot from the back of the arena). That's the only place where you actually get a front shot of Geddy's hands while he's playing the guitar.
The coolest part of the ESL video is at 7:55 where Alex seemlessly switches from his 12-string to his 6-string.
This cover video shows a guy playing the 4080/12, keyboards and Taurus pedals the same way Geddy plays them, and you can clearly see where he plays the guitar neck:
https://www.youtube.com/watch?v=WwmKfhxVNOk
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