Multiple connections to same DocType based on different field

ERPnext v14.1.2 / Frappe v14.5.0

I have customized Sales Order with a 2nd link field named end_user_company to Customer. This allows me to record an end user company for items sold through a reseller (the latter being referenced by the customer link field).

Now I’m trying to create an Document Link (connection) from Customer to Sales Order, based on end_user_company. This works, but as soon as I do that, the original Document Link based on the customer field stops working. Even if I manually re-add it, it will not use that field, but always the one for end_user_company.

Any idea how this could be fixed ?