You are correct.  There needs to be some sort of switch connection between the two core switches that allow them to be seen as a single switch by all other devices on the network.  The three most prevalent protocols used for this are Stacking, MLAG, and VLT.  Which one you select will depend on what your switches support, and then on preference.  Most switches support Stacking.  Most switches also support either MLAG or VLT, but not both.  You would need to review the switch features and specifications in order to see what each specific switch supports.  For example, most Dell N-series switches will support stacking and MLAG, while most Dell S-Series switches support stacking and VLT.  I believe the 54xx and the 62xx both support stacking only, which should work for this scenario. 

Hope this helps.

-B 

I understood. So, I can create a unique LACP per each switch N and then connect one cable to "switch core 1" and one to "switch core 2" and, as switch core 1-2 are in stack, I create one port-channel with interfaces 0/g1 and 1/g1; like:

==== switch core 1-2 ====

interface port-channel 1
description "test"
switchport mode trunk

interface ethernet 0/g1
channel-group 1 mode auto
interface ethernet 1/g1
channel-group 1 mode auto

on client only one LACP with two ports, each connected to one core switch.

correct? (because I am not sure if the network design is correct)

thanks

Nice. Thanks a lot for the explanation.