Hello Christian,
If you have ethernet swithces that support LACP, we would recommand that.
1.)What you can do is use on FAS1 ethernet 1 and 2 with LACP over one switch (LACP1) and ethernet 3 and 4 with LACP (LCAP2). (same on FAS2)
2.)Then you create a single mode vif with LACP1 and 2 (active/passif)
You will have then 2gbps redundant link going to 2 different switches.
Hope that can help.
If you use Cisco switches...LACP is the channel-group command...
Regards,
Hadrien