Second party would be whoever you’re transacting with.

In language/linguistics 1st person is your own self and second person is who you’re talking to.

Singular
1st person - I/me
2nd Person - You
3rd person - He/She/It

Plural
1st person - We/us
2nd person - You all (y’all)
3rd person - they

In relation to bitcoin custody, with third party custody, the custodian would be a third party that sends Bitcoin to a second party on your behalf.

Even joint custody would be you as the 1st party and all of the other key holders would be third parties.

Does that make sense?