If you get a CP pass (currently $89) you can add a GL add-on for $28. If you buy a CP Combo Pass that normally includes Soak City ($144), that automatically gives you the GL add-on. This much you can learn from the website. But I had to go to the season pass office during Coastermania to get my own additional questions answered...
"Is it all on one pass? Or two separate passes?" It is indeed all one pass. They do their Combo Passes and Joe Cool Club passes by putting those extra images on the pass. Similarly, a GL addon will have another image (perhaps a "G") on the pass. I hope the GL people are competent enough to allow me in!
"Can I renew an existing CP pass and add a GL pass?" Yep. You cannot do it online but you can buy the certificates at the park or by phone, and then send them in to renew your pass automatically without standing in the ridiculous line. Hooray!
And for further clarification (these are to be expected) - no you can't get one parking pass for both parks, and the Joe Cool Club means nothing at GL.
Similarly, you can buy a pass at GL, (at the now crazy price of $69) and add $40ish to that for the CP pass. Hope that clears it all up for some people!
The Cedar Point combo pass, which has always included Soak City and Cedar Point, is $144 and the season pass office considers the Geauga Lake entry a Free upgrade to the combo pass. They also have a free Joe Cool upgrade with the combo pass.
If you do the math at the current prices, for a Cp Pass, Joe Cool, and Geauga Lake, the price is $130. for only $14 more you can upgrade to the Full combo pass.
I have never been to soak city and am not a waterpark fan at all, but I purchased the full combo passes. I figured for $14 I might walk-in their next year to cool off on hot day. Who Know? I think it was a great deal.