In my opinion, mathematics and physics are interconnected; I think almost all theoretical physicists are mathematicians and many mathematicians are interested in physics subjects.
Since Codidact is an embryonic Q&A website, I suggest that one community is devoted to both mathematics and physics to attract enough contributors to advance the community. If the community were overcrowded sometime, then it could be split into two separate communities.
However, before launching the new website, the following points should be noted:
I know that the main motivation for creating Codidact was some controversy about some meta issues. But, please note that most mathematics or physics experts do not care meta subjects like "Code of Conduct", "Copyright Licenses", ... (There are many mathematicians and physicists using copyright-infringed resources easily and sometimes publicly). They are mainly interested in the quality (features, functionality, design, ...) of a community. Although I have joined Codidact since a few days ago, I have seen some bugs and some basic-features lack (For example, lacking preview feature, comments cannot be voted, flagged, or linked, showing related posts, advanced search options, ...). Comparing Stack Exchange with Codidact, why should a person, whose main motivation is to ask and answer technical posts, prefer Codidact over Stack Exchange? So, I recommend that before launching a new site, please first improve the quality of Codidact.
One of the most important things for a person to contribute to a community is the population of the community. Even those people caring meta issues like "Code of Conduct" are not motivated enough to contribute to a community with a very small population. I think putting the link of Codidact on SE profiles or in SE chat rooms is not enough to advertise this new community. It seems that the Codidact community team should find a solution for this issue.
I cannot really understand how some comments have been implied from my words, so I have to mention the following points:
The relation between (theoretical) physics and mathematics is not the same as that of economics (or finance) with mathematics at all. Many ideas in mathematics come from physics and vice versa while only a few (with respect to physics) math tools are used in economics; mathematics helps physics to grow and vice versa. Great theoretical physicist are great mathematicians.
I never said that "math is only used for physics". I meant these fields are so close to each other that one community can be devoted (in the beginning, to attract enough contributors) to cover both mathematics and (theoretical) physics subjects. All mathematics questions are welcomed there and no one asking some math involved in biochemistry would be thrown out.
I never said that I want to combine mathematics and physics subjects; there would be no combination or mixture. Saying many times, because of the strong similarity between mathematics and theoretical physics content, one place can be dedicated to these fields. I hope it is clear enough that no one wants to combine set theory with Judaism.
However, it worth noting that the two points mentioned in my original post should be considered as the main concerns; the other things are marginal.