There are a number of views on their identities brought by Da'at Mikra on Divrei Hayamim 1:18:7. The following two are the ones relevant to your question - views that these were distinct groups of people:
Cheretites - from the Cheretite Negev, mentioned in Shmuel 1:30:14, and Peletites - descendants of Pelet, from the family of Yirachme'el.
According to Rabbeinu Yonah Ibn Janach in Sefer HaShorashim: Two groups of Plishtim from Crete.
A slightly more nuanced version of the above two options is offered by the Encyclopedia Mikra'it (Hebrew Biblical Encyclopedia), entries: פלשתים and כרתי, that the Cheretites and the Plishtim are not the same people. Likely the both came from the same area in the Mediterranean, but they are two distinct groups (similar to how the Tanach differentiates between the Kaphtorim and the Plishtim). This might serve to explain the term "Cheretite Negev": The Cheretites, in order to distinguish themselves from their countrymen, settled south of Pleshet. It's also possible that the Cheretites actually made up an earlier immigration wave to Canaan, arriving there before the Plishtim. Connecting this view with what Trude Dothan wrote in her book "The Philistines and their material culture", pg. 17, that "The Philistine expansion toward the southwest is probably indicated by the term negeb hakreti", it's possible that the Cheretites originally settled with the Plishtim, and then perhaps volunteered to assist with the expansion by settling a particular area of the Negev Desert. The Encyclopedia believes that the Peletites are regular Plishtim.
In Olam Hatanach on Shmuel 2, it is suggested that per the reading "הכרי והפלתי" (Shmuel 2:20:23) instead of "כרתי ופלתי", the Cheretites may have actually originated in the Caria region in Anatolia, which at the time was known for having many mercenaries-for-hire (though the Tanach never outright states it, it seems that that's what the Cheretites and Peletites were). It's worth noting that, as I first heard from archeologist Dr. Amit Dagan, inscriptions found in Turkey tell about a King Taita ruling the "Kingdom of Palistin".
I distinctly recall seeing a number of other sources with other views, but haven't been able to track them down yet. I hope I'll be able to eventually.