As a long-time Pilgrim Office volunteer, please allow me to elaborate on some of the comments above.
1. Maintaining a second credencial in parallel may be a meaningful tribute for you, but it does not affect having a second name added to your Compostela. So, go with your heart on this suggestion...
2. The official rules state that any person can accomplish a Camino on behalf of a deceased person, or a live person whose physical condition is such that they will NEVER be able to do a Camino. No documentation of a relationship or condition is usually asked for.
3. When you present yourself to request your Compostela, you merely inform the person behind the counter that you did this Camino for the memory of or on behalf of a deceased family member, friend, or whatever...and could they please add the person's name to your Compostela.
Again, no proof will be asked for. However, it would be helpful if you could provide the name of the person, clearly spelled out in English / Roman letters, so they can write it accurately (perhaps with the first (given) name in Latin).
The staff person will write "In Vicare Pro" at the bottom of the Compostela, followed by the person's name... This Latin phrase means, in its colloquial form, "in place of..."
You do not need to volunteer the actual relationship beyond what you choose. This said, and in the case of same gender relationships, and understanding that we are talking about the Roman Catholic Church, I suggest that less said is better...
Most commonly heard requests are for pilgrims requesting a deceased spouse, sibling, child, or parent. However, I have even instructed people who ask about adding their human friends. I have even been asked about adding deceased and beloved pets. Again, without judgement, I tell them to indicate the pet's name as though it was a human, using their family name... "...eg. Peaches Smith."
For what it is worth, I have done this twice, once for my younger brother, who died many years ago, and more recently for my father, who passed in 2017. I will likely be doing it for more family members and friends in future, at least as long as I can walk.
Final point, Catholic Church dogma, applicable to practicing Catholics, states that any indulgence accruing to the soul of the person performing the required action or duty (i.e. the formal activities at the Cathedral on arrival) to earn the indulgence, or within 15 days, ALSO ACCRUE TO THE SOUL OF THE DECEASED PERSON.
So, practically speaking, this means that, as a Catholic, if you were to arrive at Santiago, and perform the actions specified for a Catholic to obtain a plenary indulgence during the coming 2021 Holy Year, your spiritual indulgence also applies to the person named on your Compostela...
I did not make this up. These last two points can be verified by checking the Cathedral website and searching for plenary indulgence ("indulgencia plenaria"). Examples of other recent grants of this sort of indulgence by the Pope will result.
The 2021 Holy Year provisions have not yet been posted. For those Catholics out there who do not know, or who have forgotten, what a plenary indulgence is, check here:
AN introduction to indulgences. What are indulgences? How do indulgences work? Why do Catholics have indulgences?
www.catholic.org
I do not make this stuff up.. I was born into this tradition, and it sort of stuck with me over more than 65 years... I don't make the news...I just report it...
Hope this helps...