'Sorry, that email address is already used!’

Hi there,
I have a multisite with two very different webshops (let's call them SITE1 and SITE2), however now I have a few users on SITE1 who also want to buy items on SITE2 (and vice versa), but they get this error when they try to sign up:

'Sorry, that email address is already used!’

I guess this error occurs because it is a multisite, but obviously it should be possible for customers to sign up as users to both sites individually.

What is the workaround for this?