I always try to leave the spliced section at least 1" thick back to belly. This way any wood I add to the handle to build it up is very unlikely to pop off. I add handle wood to every spliced set of billets I put together. One reason is to add reinforcement to the splice, the other is to have enough wood for my distinctive bulbous, locater grip handles.
My stock of billets is made up of a lot of random pieces. Often one billet of a matched set has enough wood on it for a complete handle but the other does not. I like them to match in dimensions when I splice so I bring the larger piece down to the size of the smaller piece.