Looking to fly ORD to Split b/t in July or Aug 08 on my US Airmiles and purchase a companion tix. I'm thinking the cheapest way is from ORD to London and Easy Jet to Split. Any other advice?

Firstly, welcome to FT !

Not sure if they codeshare with Croatia (OU), but if they do, then you may find ORD-SPU fares available with the following all-Star options.
LH via FRA, LX via ZRH, OS via VIE. SK also serve (via ARN) SPU on their own metal, so that's another option. Croatia serve FRA ZRH and VIE with nonstops to/from SPU at that time of year.
If LH/LX/OS do codeshare on OU, then you should be able to buy the revenue ticket ORD-SPU and also then claim your Star Reward ticket all the way through as well. Even if they don't code, they may still offer a through fare. SK should obviously offer it's own fare as it's all SK metal operated.

It's also possible UA may have a codeshare or a through fare available as well, if they have any deals with OU.

Cheers.

When I saw the heading, my first thought was get a good divorce
lawyer first....

If the intent is fly the entire way on one ticket, I think you may also be able to use Adria airways through Ljubljana, Slovenia. I had looked at doing something similar this way but in the end, decided to go directly into Dubrovnik rather than Split. The ANA tool is great for showing you alternative routes.

US Miles should get you on Croatia Airlines (OU).

Thank you for the tip. My ex, an avid flyertalker, used to take care of all of our travel plans. Now, I'm growing up and learning to do it myself.