With your cycling base I think you could incorporate some rides into the Big Vert plan without overloading yourself. During the early part of the plan where the emphasis is on the ME workouts and easy base miles you would probably be advised to keep the cycling fairly flat and mellow as cycling (especially hilly mtn biking) already has a large ME component baked in. If you hammer you legs in the plan’s ME workouts and then again a day or so later on the bike you may be overdoing that ME loading and not see the expected progress. Late in the plan after the ME phase would be the time to begin to add some intensity in the cycling. At least that’s the way I think I’d handle this back to back event. The BV plan should do a good job of making your legs more fatigue resistant so you will be starting the second day on the bike a bit fresher.
Good luck. Sounds tough.