sounds just like my setup with the tie bar, a 225 yamaha ox66 & 9.9 kicker connected with a diy tie bar. I went with Lowrance Outboard AP hydraulic and went with the new HDS live unit + bluetooth remote steer. Its a slick setup!. Installation was Easy done in the driveway. Tyee's advice above is spot on about matching your existing electronics if you can.
The AP works with both motors as my setup is always connected. The motors don't need to be running for the AP to work.