It's not uncommon, and it doesn't always work the way people imagine. I've seen it where yes, a provider does have someone on speed-dial. And I've seen clients pay the rate for another client so their fantasy could be fulfilled.
Also, to be fair, that what you're describing sounds pretty close the Vixen/Stag dynamic which is different from cuckolding. Traditionally cuckolding, the arousal is typically tied to humiliation, powerlessness or intentional exclusion (you can watch but you can't touch or you can't watch at all but I'll tell you about it). The "cuck" is meant to feel lesser, sidelined, or even emotionally displaced. The whole "loss of status" is park of the kink.
With the Vixen/Stag dynamic, which is where I play, that dynamic is not about humiliation or displacement. The Stag is intentional, confident, and in control. Their pleasure sometimes comes from watching, orchestrating, or later reclaiming the Vixen (re: your "finishing" up comment). There is no exclusion.
I'm just making an educated guess that you're aligned more with the Vixen/Stag dynamic because based on your post, it feels like you're centering the experience on pride, agency, and ownership of the situation... not humiliation and diminishment. Feel free to correct me if I'm wrong.
