I agree with the other post this isn't possible unless your can do link aggregation between the switches (LACP). A common example is with Cisco 3750 switches that use a stackwise cable.
The other option would be to create two LACP VIFs (one per switch) and separate your traffic so at least you are utilizing all four interfaces. For example, create VIF1 and use Switch1 for CIFS traffic. Then create VIF2 and use switch2 for NFS traffic. This is assuming that you have two protocols on your FAS2040.