If you pay €6k into the account of A (and there is no contract, employment relationship, etc to explain the payment) that looks, on the face of it, a lot like a gift to A.
As Tommy says, it's unlikely that the Revenue would take much interest in this, even if you do it every year and the total amount paid in aggregates to many tens of thousands.
But, if they do take an interest, and your explanation is that this is actually a gift to A and his partner B jointly, the onus will be on you to show that paying money into A's account was in reality making a gift to B. As the payments will, by this point, be in the past this really comes down to you showing not only that B did get the benefit of 50% of the money paid into A's account but also that, when you paid the money, you knew that this would be so — i.e. you had an express arrangement with A about this. And the Revenue will be the teeniest bit sceptical, since the obvious way for you to make a gift to B would have been for you to pay the money into an account belonging to B. So their first question will be "why did you not do that?"
The case linked in Sue Ellen's post makes the point that you can't make a gift to B by mentally "earmarking" money in your own account for B; you have to actually give the money. But I think if a suitable case arose we'd see a ruling to the effect that it's not enough to give the money to some random person who will mentally earmark it for B; you have to give it to B, or to someone who is bound (as in, "is obliged to", not "is likely to") to pay it on to B, or pay it on as B directs.
So, bottom line: do you have to pay the money into B's account? No, you don't absolutely have to. But why in God's name would you not?
As Tommy says, it's unlikely that the Revenue would take much interest in this, even if you do it every year and the total amount paid in aggregates to many tens of thousands.
But, if they do take an interest, and your explanation is that this is actually a gift to A and his partner B jointly, the onus will be on you to show that paying money into A's account was in reality making a gift to B. As the payments will, by this point, be in the past this really comes down to you showing not only that B did get the benefit of 50% of the money paid into A's account but also that, when you paid the money, you knew that this would be so — i.e. you had an express arrangement with A about this. And the Revenue will be the teeniest bit sceptical, since the obvious way for you to make a gift to B would have been for you to pay the money into an account belonging to B. So their first question will be "why did you not do that?"
The case linked in Sue Ellen's post makes the point that you can't make a gift to B by mentally "earmarking" money in your own account for B; you have to actually give the money. But I think if a suitable case arose we'd see a ruling to the effect that it's not enough to give the money to some random person who will mentally earmark it for B; you have to give it to B, or to someone who is bound (as in, "is obliged to", not "is likely to") to pay it on to B, or pay it on as B directs.
So, bottom line: do you have to pay the money into B's account? No, you don't absolutely have to. But why in God's name would you not?