FG Manage two FS with a dell switchL2 between

Hello everyone,
Is it possible to manage two FSvia a FG but between the two there is a Dell switch Layer2
The FG is connected by fortilink to the first FS. The first FS is connected in Trunk LACP to the Dell Switch layer2. 
The Dell switch has a trunk link to the second FS.
Everything's fine except that the FG can't connect to the second FS. The second one doesn't appear in the FG platform.
Thanks for your help