For same-sex couples the route question and the legal question are the same question, and the answer changes at the border.
Two mothers
The practical routes are clinic donation or a known donor. The legal question is how the non-carrying mother becomes a legal parent — and this differs sharply between countries. In some, marriage or registered partnership does much of the work automatically. In others, a separate step is required, and skipping it leaves one parent legally a stranger to her own child.
That step is not paperwork to do eventually. Until it is done, the family is one accident away from a position nobody intended.
Two fathers
The route requires a woman who carries the child, whether as a co-parent or through surrogacy, and surrogacy law across Europe ranges from regulated to criminal. This is the area where cross-border arrangements are most common and most legally fraught — a family formed lawfully in one country can find its status contested in another.
The known-donor question
Many same-sex couples want a donor the child can know, and often want him to have some presence. That is a defensible choice and a legally delicate one: presence can, in some jurisdictions, become the basis of a claim. Wanting a donor in the child’s life and wanting him outside legal parenthood are compatible aims that need to be arranged deliberately.
What to do first
Before conceiving, not after: establish how the second parent acquires legal status in your country, and what the donor’s position is. Both answers are country-specific and both change. Take advice where you live, and again if you move.