I have a spouse partner to whom i am legally married and have been with for 13 years. Spouse and i each have one other partner. They have been together one year and i met my other partner in 1988 or 1987 and we have had a thing all along but sometimes years go by where we have no contact at all (i don't want that to happen anymore but that doesn't mean it won't), but we don't "break up". I only see Other Partner 3 or 4 times per year, because that is how often the planets line up so that we both have free time at the same time. So, i guess i could answer your question like this: when i have the opportunity to spend time with Other Partner and Spouse is also available, i would prioritize hanging out with Other Partner because our time together is so limited. In no way does this reflect upon how much i love either of them nor is it because of a "rule" or some "list" that i keep. I think of priorities within each relationship, not priorities of the two relationships relative to each other.
I think the reason people tend to have strong opinions about this "hierarchy" business and worry about "what if i'm not as good as my partner's other partner" comes from being conditioned to be competitive since the time we were small children. I can remember being drilled with things like, "we can't all be good at everything. Find what you're good at and be the BEST.". While this looks good on paper, it still sends the message that if you're not the BEST at something, you somehow failed altogether. It took me a couple of years in my early 20's to figure out that i am non-competitive by nature, which can also work against a person because employers often equate non-competitiveness and lack of ambition with laziness and apathy. But this is not about me. I'm just saying, these traits can have an effect on more than one facet of life.