Exim Filter to change recipient

Lets say I have this filter for user1 who doesn't exist on our network, but has an exteal email we want to send the emails to:

if
 $header_to: matches "user1@domain.com"
then
 deliver "user1@extealdomain.com"
endif

If someone then sends a mail to user1@domain.com it correctly gets delivered to user1@extealdomain.com

However, if they send to user1@domain.com, user2@domain.com then the email only gets delivered to user1@extealdomain.com

user2@domain.com doesn't get sent the email.

I think this is because the deliver command is significant, so nothing further happens.

Normally, you would use unseen deliver to ensure the email is processed further - but then this would (I assume) result in the email being delivered to user1@domain.com locally (in addition to user2@domain.com), which isn't wanted.

What is the right way to set up this filter so that the emails will go to the exteal party and the inteal user if they are both in the To: field?