There is not just one right way
If you are interested in your best friends spouse I think who you tell first or if you tell both at once depends on the nature of your relationships with both. For me, I know that if I had a best friend whom I considered was a lifer I would feel a need to talk to that friend first before pursuing a crush on his spouse or GF. I would do this because of my own need to keep/maintain that relationship (friendship) as a higher priority than pursuing the crush I had on his/her partner. This is similar to my need to talk with my DH before pursuing any crushes I have because I place a higher priority on maintaining my marriage and checking in with my DH’s feelings before I act on something that doesn't yet exist but could change the nature of our relationship if it did.
If I know that I definitely would not pursue an interest in someone if a partner of mine (friend or romantic) was very uncomfortable with it, I would definitely go to that person first. I don’t know if this is the right way or not but I would see it as the best way for me to approach the situation. Perhaps this is because I am a VERY loyal person to those who I am in relationship with and I would want to check in with those relationships before doing something that I believed might damage them.
If I know what my actions will be based on a specific person’s feelings then I don’t think it is required to inform the crush beforehand. I guess I would be making decisions for this fantasy relationship but those are MY decisions to make based on the current relationships I have at the moment. If they are secretly yearning for me it is up to them to voice that to me or their partner. They can make their own choices too. I consider my committed friendships and romantic relationships before I consider crushes and would-be fantasies. This is because I can choose not to act on my crushes if I feel that they will damage my existing relationships. Why potentially hurt/involve the person I am crushing on if I know I will shut it off quickly if a partner of mine (friend in this case) objects strongly to the point of it damaging our relationship/friendship.
I think this only works if the feelings are just a crush and not fully developed yet. Once I cross to “in love” this path might not work for me.
In a different scenario where I am equally enmeshed and friends with both equally I may choose to come out to both at the same time in a light way, “Hey, I know we are all good friends. I totally love being in both of your lives. Lately, I am finding myself crushing on Y. Is it totally crazy of me to entertain these fun thoughts of becoming more especially since we are all such good friends?”
If I am having “Love” feelings for one person and I feel I NEED them to know then I might do the letter writing- one written specifically to each. If I wanted to share these feelings in person rather than writing I probably would share it with my friend first as a heads up while also stating my intention to share it with my love (his wife) shortly there after. Yet again, depending on the feelings I have I might NEED to tell my Love first and I would hope that my friend would understand. I definitely wouldn't keep them in the dark too long.
If I had deep feelings of love I would feel uncomfortable having a group conversation right out the gate (especially if I am not yet experienced in group relationship conversations). I personally wouldn’t want anyone else present if I were to be saying I love you to someone for the first time. Nor would I want anyone else present if someone were to tell me they loved me for the first time. To me some things are best left to one on one moments. I would hope both your friend and your love would be understanding of you wanting to talk with both yet separately at first. Just like you would be understanding (I hope) of them wanting some time to speak to each other alone initially.
I would ask myself, “What do I need to move forward in the best way for me?” based on what my relationship goals are at the time. If I would feel bad/guilty not talking to your friend first do that. If you don’t want to pursue a romance if your friends not ok, do that. If your feelings are too strong and can’t be shut off, consider that. What does your body and core tell you? This is where your answer is I think and it’s ok if it is different than what other people would do. Their relationship goals and modes of operation/prioritizing might be very different than yours.
Hope this helps,
P.S. I personally wouldn't feel slighted at all if my DH's best/good friend brought up with him first a romantic interest in me- to check his radar before pursuing/talking to me. It would show me that this friend values existing relationships and wants to communicate with people who are important to him before making big decisions/changes. To me friendships are equally important as romantic relationships are. I would definitely pass on certain romances if I believed they would damage my core relationships. I would respect the friend's need to make sure his friendships don't get damaged over mere possibilities and I wouldn't take it personally. It would be the same as me checking in with my DH before pursuing a relationship with his best friend.