Hoo boy do these two have a backstory. So let's start with York's life. He used to be a human in a comic-book heroics-styled world until a group of villains attacked his high-school with a mutagenic weapon called "Mind's Eye" which violently mutated the majority of the school's population. York wound up as a fire-breathing dragon, and then had to finish high-school as such.

After high-school he joined up with a local super-team, alongside characters like the powerful PowerSauce, the ultra-stretchy Yoga Man, and Snapshot the selfie-taking hero. During one of their adventures they encountered BadMerlin, a technomagician who attacked the team with the power of his Irony Machine. PowerSauce became weightless, Yoga Man was shattered into pieces, and Selfie was flattened like a sheet of paper. York meanwhile... wound up with a second head that sprayed him with water whenever he tried to use his fire powers.

As the team tried to figure out how to use their detrimental forms to fight BadMerlin, York tried to first gag his second head, then distract it, then just yelled at it (which got him sprayed with water) riiight until his second head accidentally collided with a sharp corner and started crying, which caused York to instantly react protectively to the poor crying thing. He spent the rest of his time with his new head by exposing it to pleasant things like music and ice cream, and eventually named him Izzy. Eventually he was able to light a fire in front of Izzy without him spraying it out, although communication was still an issue as all Izzy would say was "Glrgrl" type noises.

Just after the group defeated BadMerlin and figured out the 'reverse' function on the Irony Machine, Izzy made a 'Yark!' noise at York, and York realized that if he was turned back to normal, Izzy would essentially cease to be. So while the rest of the group activated the reverse, York dove out of the way (much to the surprise and shock of the rest of the group) to keep Izzy safe.

And so with his newfound conjoined brother, York left the team to help Izzy grow-up and mature a bit, learning about the world around them. Eventually Izzy learned how to talk and got smarter, and their relationship developed more into a proper sibling relationship. They now work together as a fire and water spitting superhero for hire.

...things got interesting later when York, who is gay, realized that Izzy was straight.