OS X El Capitan find mouse dosen't work with external mouse
I know a hack to make it work. It's called "tell Apple that there's a bug in their beta software."

Seriously, don't expect flawless performance from features that still have every right to be half-baked. Open Feedback Assistant and file a report in the hope that Apple will have it fixed by the time El Capitan gets its stable release. Odds are other beta testers are having your problem too, so it's your duty as a tester to report all the bugs you can find.