'DD1 & DD2's bedroom' or 'DD1 & DD2s' bedroom'?

[Readytomakechanges]

Please can anyone help me place the apostrophe correctly in the name plaque that we are making for the bedroom door of DDs 1 & 2?

Thank you.

Let's assume they are called Katie and Emma

It would be Katie and Emma's bedroom

The first. Because there's only one bedroom.

I think

Thank you.

It's the first, but not because there's only one bedroom. The apostrophe refers to how many owners there are, not how many items being owned.

It would still be Katie and Emma's toys, for example.

I think Katie's and Emma's room is clearer, and more symmetrical.

That way, these sentences have different meanings:

In the sitting room, you'll find Katie and Emma's toys. (K is in the sitting room, toys belonging to E are in the sitting room)

In the sitting room, you'll find Katie's and Emma's toys. (Toys belonging to K and to E are in the sitting room).

You only need an apostrophe on the second name. If you put one on the first as well, it means there are two different rooms.

No, it doesn't. To my ear, Katie's and Emma's room definitely means one room belonging to both.

My DDs shared a room for years. Their door plaque just said Katie & Emma (but their names, obviously)

www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv57.shtml

Rule 4b explains it clearly

www.grammarbook.com/punctuation/apostro.asp

If there were two different rooms then rooms would be in the plural. Katie's and Emma's rooms.

Exactly, River, rule 4b in TLIGD's gives an example where the object being possessed is plural. If room is singular it must refer to one room, and whether you put 's on the first name is a matter of style and clarity not grammar.