Submitting two articles to the same journal at the same time
It is absolutely unremarkable to submit two related papers to a journal in parallel.
In fact, it is often better to have them submitted to the same journal than to separate journals, so that they can be handled as a "package" of related work by the editors.

My one key suggestion is to make sure that you explicitly call out that you are doing this in the cover letters for the papers, and to have the papers refer to one another in their related work sections. This will give you a chance to make your case to the editor and reader that the papers are indeed each able to stand alone.  Moreover, if you are up front about this and the editor disagrees, then they are likely to ask that you merge the papers rather than rejecting them or getting upset about "salami slicing".