You probably should use MLAG. You configure the X620s as MLAG peers and let them act as a single unit in regards to the LACP LAG they present to the access switch (X440). This is the preferred method for large customers in the core and distribution layer unless they use more advanced techniques like MPLS or eVPN.
You'll find examples in the EXOS user's guide. Consider using a LAG as the ISC connection between the X620s and/or an "alternate ISC" that enables signalling if the ISC fails. You don't want a single link between the MLAG peers (X620s) since if that link fails, they will start forwarding frames in an uncontrolled fashion causing wire speed loops.
Good luck!
/Fredrik
