I did it solo last year for the first time, was relatively fit but hadn’t done a lot of training on the bike. As has been said there is a lot of climbing and a hardtail will always be faster for this. If in doubt do the same loop a couple of times with each bike on different days and see what the difference between your times are. I did it on a 100mm anthem as well last year but have since bought a stumpy carbon hardtail 29er. My strava times for the climbs at GT have come tumbling down on the hardtail. I’d imagine pairs would be different to solo but no easier, as you are going flat out for the lap you are doing where solo you have to pace it slightly more.
In terms of training I’d put a gym session in there once a week as well. Compound moves such as squats, deadlifts, kettlebell swings along with lunges, leg extensions and calf raises etc will help with climbing and overall fitness massively along with the rest of your training.