How to configure XG550 for LAG to a pair switches Cisco

Hi,

Recently we have acquired a couple of sophos xg 550 for our company.
The idea is to have each xg in a different location for backup. I attach a network diagram.

In site B (to be done) only the firewall and  switches will be active for backup. The servers and other equipments will be in "stand-by" and  only will be activated in the event of Site A goes down (thus, in case L2 line goes down there would be no problem if the XG550 in SiteB is activated)

Now, i want to connect de XG550 in Site A to the two switches Cisco (stack1 and stack2). I have configured the two ports 10GbE from XG550 in LAG and connected PortB1 to the Stack1.

This is the configuration on switch side. Only trunk mode, no etherchannel.

interface TenGigabitEthernet1/0/23
description Conexion con Sophos XG550
switchport trunk native vlan 17
switchport trunk allowed vlan 17,240,241,421,502-650
switchport mode trunk
end.

This way everything is working properly. Now the question is, Would there be any problem connecting PortB2 to stack2 if I configure interface TenGigabitEthernet1/0/23 in Stack2 in the same way it is on stack1 (only mode trunk) ? Or is it necessary to configure etherchannel in the switch side?.

I know this is not a sophos operation but let's see if there is someone out there who has a similar design and can give me their feedback. 

I have not found any configuration examples anywhere. I would appreciate it too.

PS: Excuse my inglish

Regards,

Roberto Sánchez.