How can I add single point of contact to multiple accounts?

How can I add single point of contact to multiple accounts?

Hi guys, 

I would like to add a single contact to multiple accounts. Here's the scenario: I have 2 accounts in my CRM and Desk as well. Such as; Company A and it's sub-company called Company A2

There is only one contact for these both companies. Contact X. How can I add Contact X to Company A and A2 at the same time?