A picture is worth a thousand words, so here's two:
You can sometimes reduce the leaking by setting input priorities (on either splitter, interestingly), but this isn't always an option, and is inconsistent in operation.
In the interim, I would recommend anyone trying to build logic gates out of splitters leaves at least 1 belt space between them
[kovarex] [0.16.51] Nested priority splitters leak
Re: [kovarex] [0.16.51] Nested priority splitters leak
I couldn't reproduce this specific problem anymore.