Uh, hello! Long time no see... Holidays fault!
This is pretty lazy, I know, but it's more like a... Characters design on chibi maybe?
So yeah, I came up with somewhat Apocalypse Au! Basically, zombie apocalypse came to earth (cheesy, I know) and those two team up to stop this.Ah, yes, Shiki's are not in zombies taste (they're dead after all, duh) but it goest either way and Shiki's want to eliminate them as they, well, kill people.
So, since Tatsumi is Jinrou, zombies are only half interested in him, he's immune to zombie virus but he still can get killed.
And, Toshio is basically brain of the duo but he still can defend himself (with a baseball bat, of course).
Tatsumi believes that Toshio is smart enough to find the cure and Toshio is using Tatsumi as a additional force. Strangely, their alliance is hugely based on trust (to this point when Toshio don't even look up from his science in big fights, letting Tatsumi do the work) as they realize they'd be very much fucked without each other.
This don't stop them from teasing and arguing though.
Basically, I came up with this as I thought "Wow, how cool it would be if they team up!".
(Toshio's cap is strategic while Tatsumi's being fancy as always.)