Setting Up Multisite For Two Different Domains (With a Twist)?

Hi, I would like to setup multisite for two (possibly more) different domains (sitename.co.uk and sitename.com), mainly for membership purposes. I want users who register on sitename.co.uk to able able to login with the same username & password on sitename.com, and vice versa.

Here’s the twist, the two sites are hosted on two different servers/host providers. Reason is for SEO purposes; sitename.co.uk is for UK market (hosted on UK servers) and sitename.com is for US market ( hosted on US servers).

How can I setup multisite for these domains?

Thanks