Game
It was all a game to them. One of them creating Jutsu, the other banning them. It hadn't started as a game, but to the casual observer it almost looked like it.
It had all begun with the now infamous Edo Tensei Jutsu, which had been easy enough to ban. Why on earth would you even invent that? Hashirama had asked his brother.
Next had come Hashirama's Mokuton style, which hadn't so much been banned as it was Tobirama telling him to stop messing around and focus on the paperwork.
After that Tobirama had been messing around with the standard clone jutsu, attempting to make them tangible, when he had accidentally created the Shadow Clone, which while not having solidity, it had had sentience. Hashirama had banned that particular Jutsu partly because of the massive amounts of chakra it consumed, and partly because the multiple Tobirama's were getting creepy.
And for the pair it continued for years, each creating small and often pointless jutsu, and the other giving reason they can't use it.
I have no idea what the fuck I just wrote.