Hi,
you need setup vPC doman on Nexus (connect 5K1&5K2 to each other via Peerlink and PeerKeepalive link, configure vPC domain). Then on each Nexus port to Fortigate configure as following:
Interface Eth1/10
channel-group 10 mode active
interface Port-channel 10
switchport mode trunk
switchport trunk allowed vlan x,y,z
vpc 10
Vlans x,y,z must be allowed in Peerlink.