In animation, when we're adding a new version to a timeline, we vary the opacity of the new clip on the higher layer so that we can see both old and new at once. You can then tap though with the arrow key or play to see if they're in sync.
You can also resize images and put them in different corners of your Program Monitor if you want a clear view of what's going on.
Or make an adjustment layer at the top, apply the 'Timecode' video effect for however many simultaneous clips you're using, and point each instance of that to a different layer of the timeline so that you've effectively got a BITC for each layer.