So, I was doing some more labbing today trying to test out stuff and I decided to figure out if I can make Cetrion's geyser not randomly whiff the second hit on Geras and Kotal in combos. What I found was that I could 100% consistently get the second hit to work on him off the 213 KB (most important place for it to work, honestly). You have to instant j2 him after the pop up, which makes him fall very close to the ground before f13 hits him. As long as Geras is close to the ground, the second hit of geyser will pick him up. So when doing combos try to do your jump ins early enough that Kotal/Geras stay very close to the ground. If they're too high up, meaning you did the jump ins too deep, it will drop because the second hit whiffs.
Edit: Adding a video to show what I mean, the last combo is doing a deeper j2 rather than an instant for the whiff.
Edit 2: After more testing for normal BnBs, the j2 j1 combo you have to have somewhat specific timing to prevent geyser whiffing, however, the BnB with double j2 does not have this problem, so I would recommend getting down the double j2 instead of j2 j1 vs Geras and Kotal since it always keeps him low enough for Geyser to connect properly.
Edit 3 because I hate multiple posts:
The double j2 is consistent on Kotal as well as Geras and below is a gif of j2 j1 whiffing because of height.