You need to use the Timeline feature of Captivate and adjust them according to time. If you see both at the beginning of the timeline, they both show immediately. So you need to drag one of them to the right so the start time changes from immediately to a later point in time.
Synchronizing with audio should be a fairly easy process, but will require some trial and error to get it spot on. Listening to the audio, tweaking the slide. Rinse and repeat until you are happy.
I already used the timeline. I have two png graphics, two next,back buttons. I drag one of them to the right just how you explained it. It seems to work inside captivate when I preview the slide but if I press F12, the demo shows only the first to the left or both if They appear on the left.
My guess here is that you have a conflict with a Button object pausing when you don't expect it to.
Please post an image of what your Timeline looks like and we might confirm that and guide you appropriately.