I think that the couple you have asked already will expect there to be other Godparents.
I am a Godmother to 3 children. All of the children belong to friends. My husband is a Godfather to 1 of the children too. In all cases I have been the only friend asked and the other Godparents have all been family. I have never felt less important than any of the others because they are related, but have just felt immensely honoured to be asked to undertake such a significant role in the child's life. In response to your post I am sure that the couple you have asked will feel exactly the same pride and not consider rank order or anything like that.
My DD has 3 Godparents- 2 friends and my DH's not so 'D'brother! He has always been a bit jealous of our little unit and this has led to him being quite spiteful on occasions and arguments thereafter. We asked him to be a Godfather to try and strengthen family ties a bit and allow him to feel included, but it didn't work out like that. After a few months of the Christening he lost interest, started the nastiness again and now does not even acknowledge our DD- even if she is in the same room as him.
He has let us down greatly since he is family and perhaps rather wrongly associated with that, we did consider it would be him we could most rely to behave in the interests of our daughter. That doesn't mean to say we considered him the most 'important' Godparent but we did think he would take the role the most seriously.
The other Godparents my DD has are wonderful and she has a really special bond with them.
Our DD has 3 male & 2 female GodParents - the extra male was more of a happy accident as he is the long term partner of my Gay friend who was chosen as DDs Godfather, he is the most religious of the group, so on the day he ended up standing at the font with his nervous boyfriend & got roped in by the Vicar - she did check with us first, as wasn't sure if it was meant to be & we agreed - IYSWIM - so he became an unexpected extra GP, but he & his partner have been totally brilliant GPs & really take the whole thing seriously
Choosing GPs is a strange one, we tried very hard to chose people who we trusted to want to always be part of DDs life - one F was a very long time friend, who worked in childcare & had 2 of her own & no health problems etc & also very spiritual, so she seemed like a really safe bet - sadly turned out not to be the case at all - She seemed thrilled to be asked, yet didn't take that much interest in DD, bar on her terms - ie we take DD to meet her etc - I always thought she was great with DCs, but she was pretty brusque with DD & DD grew up not to like her much, a couple of years back, I had a very odd fall out with her - one that with hindsight I think she orchestrated as it was just plain weird - we've never heard from her again - she's cut DD off completely, if anyone had warned me how it would turn out I would never have believed it, she out of all of them really was my top choice & safest bet
Thankfully the others have all been brilliant - & the 3rd guys wife now is DDs chosen surrogate GM
So, with us there didn't seem to be any protocol as such - I did speak with the Vicar, though she is very forward thinking & she just said the more the merrier & I let the others know too - with hindsight having more has been a real blessing
my ds has three grandparents, two men and one woman - an aunt, a cousin and a friend. I thought you were supposed to have three, 2 males and 1 female for a boy and 2 females and 1 male for a girl? We told them who we had asked/were going to ask and none of them knew each other very well (still dont)
It is OK to tell prospective godparents (after you have asked them, and they have accepted), that there will be other godparents too? We have already asked one couple, but would like to add a third godparent into the mix. Keen that all should feel equally "important".