Mixing magical sources

Spoke with someone who actually has magic from two sources. One is a warlock pact, similar to my own, but to what he referred to as an abyssal one. Not sure what it is but he also Called it an Old One, so maybe it's even older than a regular god? Anyway, the other is being a worshiper of Runethare, so a priest I believe. Definitely interesting, considering how where I used to live views warlocks. But interestingly, while the sources stay separate, he can synergize and use both magics, however that works. When I asked him how it felt to use each one, he said the Arcane was more calculation based, whatever that means. I understood more of what he meant when he spoke of Runethares and beseeching the gods for aid. We spoke of the nature of being a warlock, with pact magic and whatnot. It would seems he has a far greater understand than I of what that does or does not mean. There's still much I don't know about magic. But I intend to find out all that I can.