The fact that the two films will be released within a year or so apart from each other lends to the possibility of the two being filmed back to back but it isn't only that. Most films in the same series have usually a break between them of two years at the minimum, take the current Star Wars trilogy which are each spaced two years apart (with the solo Star Wars films in between).
Another thing is the announcement today of a writing group to work on Godzilla Vs. Kong, consisting of Terry Rossio, Patrick McKay, J.D. Payne, Lindsey Beer, Cat Vasko, T.S. Nowlin, Jack Paglen, and J. Michael Straczynski. Godzilla: KOTM is only slightly ahead of this with writing being worked on and the director, and a few actors being named. It wouldn't surprise me to see this group help with rewrites that surely will happen with Godzilla: KOTM.
It would completely make sense to do this and give a flow from one film to the next given they will most likely take place in the same time frame unlike Kong: Skull Island and the first Godzilla film. It could also ensure the actors in Godzilla: KOTM film return for the next film, something which may not happen with the Godzilla actors for Godzilla: KOTM.
While this is pruly speculation only time will till if this ends up being true.