I suspect that the Isk have no mind other than what is required to keep the heart going, lungs breathing (maybe) and other such nessecary base functions. The yoort then acts as the Isk's brain.
As for how they die if they stay away from each other to long, I would do it with hormones. I would make it so that the presence of the yoort causes a vital hormone to be produced in the Isks very primitve brain that it needs to live, and I'd do the same with the yoort. Modify the yoort to require a synthetic hormone that the Isk's brain naturally produces. The yoort would then need this hormone at all times except when swimming in the yoort pool, which has been modified to contain this hormone in chemical form.
I have no idea if this is how it is done, but that is how I'd do it.