It's really not.
There is a lot of fan assumption that was undone by this show, and their poor fan head canon. But, as was noted elsewhere, Palpatine never says who was his master; we assumed it so but it isn't out and out stated. So, what lineage is leading up to Palpatine is unknown.
Two, Qimir and Osha training together outside of Plaguis blessing is hardly new Sith behavior. They're constantly seeking to Undermind and overthrow and test the other's abilities. In the original books around TPM Palpatine discusses sending back up for Maul to ensure he succeeds. That Qimir and Osha would likely die is exactly why I find the story so interesting. They got what they wanted and its a complete tragedy. It's excellent.