You can do either - or both. It depends mostly on your port availability and licensing. If you have the ports to do both, you could do that as a method of segmenting traffic.  I googled and found this reference from Dell TechCenter  www.google.com/url;rct=j&q=&esrc=s&source=web&cd=1&ved=0CC0QFjAAahUKEwiiu5j-p_DIAhUJYyYKHWqpCic&url=http%3A%2F%2Fen.community.dell.com%2Ftechcenter%2Fextras%2Fm%2Fwhite_papers%2F20439304%2Fdownload&usg=AFQjCNElhbltW0JL_8ZM5LxP8IgUSUeLrg&sig2=OFo6U6QRiNPZWVOjUivPBw&bvm=bv.106379543,d.eWE&cad=rja Normally when mixing two fabrics - like iSCSI and FC, you can justify the use of them as tiered providers in multipath round robin (so that one takes over for the other when a fabric failure occurs) - in this case you would be using (if I understand your environment correctly) the same switches to make up a single fabric. I am not sure what you would gain except additional bandwidth. If the system is designed to be very fast and low latency - that might be useful. But the main reason for multiple front ends is typically either mixed topologies or multiple front end fabrics for segmentation. 
Hi,  I am planning to deploy Dell SC8000 with cisco UCS infra. the list of equipment is below: 1: Cisco Nexus 5548UP universal license 2: Dell SC8000 SAN (4x FC and 4x FCoE ports) 3: 2x Cisco Fabric Interconnects 4: Cisco UCS blade chassis  Please recommend where should i connect Compellent in the above setup either on FI or on Cisco Nexus..? Also there are 4x FC and 4x FCoE ports whether i should go for FC only or FCoE protocol or BOTH to get the max throughput from array.?  Thanks 
HOW to Connect Dell SC8000 with CISCO UCS Infrastructure