Presumably that would affect only departure? (I ask on the behalf of the OP because open jaws are being suggested.)

I've flown BRU-ORD several times, and have never had problems with security lines. I did have a bit of delay last time because their security caught a mini-scissors I didn't realize I had in my suitcase (it was inside a sewing kit I'd picked up long before at a hotel), said US didn't allow scissors, and then turned my (poorly packed ) suitcase inside out to find them. But even that, tho it felt like forever, only took a dozenish minutes.

One thing to be aware of at BRU is that there is a secondary security check (generally ID and maybe a couple questions only) at the far end of the gate area where the US departures are, and once you pass that you cannot go back and forth easily. But it takes maybe half a dozen minutes to trudge there from the lounge area. So plan accordingly.