Why are my two websites swapping rankings on Google for the same keywords?

I have two websites offering digital marketing services, mainly focusing on SEO-related keywords like "SEO companies," "SEO agencies," etc. Both sites use the same brand name "Infinix" but have different domain names: Infinix.in and Infinix360.com.

Initially, Infinix.in was ranking on the first page of Google, while Infinix360.com stayed on the second page for a long time. Later, they swapped positions — Infinix360.com moved to the first page and Infinix.in dropped to the second page, almost exactly swapping their previous ranks.

Today, they swapped again — Infinix.in is back on the first page, and Infinix360.com moved back to the second page.

What could be causing this constant swapping between my two domains? Is it algorithm testing, keyword cannibalization, brand confusion, or something else?

Also, how can I bring both websites to rank on the first page together? Would love to hear expert suggestions or if anyone else has dealt with a similar issue!