The way Rav Shach explains a difficult Rambam in Laws of Sota perek I. Law 3 is this. There is the case of mikve lacking 40 seah. All unclean. And a barrel for maasar found to be vinegar. That is a doubt. So Rav Shach says both come from the same law of two hazakot against one. The point being that two hazakot against one is thought to be a doubt. This idea would explain the Rambam perfectly. But as for the gemara it does not seem to fit very well. Look at Rav Shach explanation of the first answer of the Gemara in Nida page 3 and the second. This answer of Rav Shach says that that Rambam borrowed part of the first explanation with part of the second to come up with a third explanation to explain the Rambam.
I do not have energy to write out the whole subject. Just look there at Rav Shach on Sota I:3 and you will see what I mean that the end of that idea of Rav Shach certainly fits the Rambam perfectly but the question is how does that for with the Gemara? There is the fact that the Rambam is posek like the mikve and the barrel. One very good idea to explain this is that both come from the idea that two hazakot are thought to be a doubt. But when the Gemara itself wants to answer this contradiction it comes up with two totally different explanations. Nida page 2 side b. This has bothered me for weeks. I van not see any answer when I look at Rav Shach. But on the way to the sea and back I ponder this question and hope that someday I may understand what Rav Shach is saying.
