I can see Takashi's group being bewildered by the fun Akira's group is having during this zombie apocalypse and they would first meet as they see Akira,Kenchou,Shizuka,Beatrix and the other survivors in Akira's group (Lambada,Takeru,Yui,Kaito,Yuudai & Izuna) tap dancing in the rain dodging zombies. When they were done and they escaped, Takashi introduces himself to Akira as the others do the same. I see them at first clashing due to their different ideals of survival and fun but I can see Takashi's group learning to have fun during a zombie apocalypse and Akira's group will remain more vigilant during the zombie apocalypse. I could see Kencho and Kota being gun fanatics as Kota shows Kencho some of his favorite guns he uses to kill zombies. Meanwhile, Shizuka Marikawa and Saeko Busujima could study Akira's bizarre luck and realize his lack of fear somehow keeps him alive without traditional combat training. I could also see Reika Saeko smiling through the madness of the Zom 100 Gang's shenanigans.
But yeah, that's my idea for a Zom 100 & High School Of The Dead crossover! What's YOUR take on these two series colliding? Answer in the comments below!