That's entirely possible. With the timeline, the photos of him coming in apparently without a rifle seem to be about 10 minutes or more before the shooting started. That's plenty of time to come in and make sure that his intended target was there before going back out to get the rifle.
The reports of a second suspect come from early witness reports on the scanner that said that he'd been seen before the shooting talking to a balding man in his 50's near the AMC theater. No word on how that panned out. Could be nothing. Could be everything.