I don’t understand why you would have gone down this road. You could certainly have kept everything together. And depending on how much history you’ve amassed in your 10.2 instance, I think it’s worth considering starting those projects over in your “historical” instance. (Note that in Enterprise Edition($$) you have the ability to move projects from one instance to another, assuming they’re on the same version.)
Nonetheless…
You say you can only run one instance at a time. Why is that? I suspect you get errors at startup of the second instance about not being able to bind to ports…? (If you’re not sure, take a look at your server logs.) Take a look at $SONARQUBE-HOME/conf/sonar.properties. Those ports are configurable.