While technically possible, I do not think this is supported. To configure it however, you'd just have to configure appropriate igroups and maps your LUNS to them.
Re: Simultaneously accessing LUNs via iSCSI and FCP (different servers)
Should be complete fine -- a LUN is a LUN is a LUN on NetApp (one of NetApp's advantages). I've done this as well without issues actually.
You'll just need to make separate igroups given the igroup is tied to the protocol (FCP or iSCSI). I'd also keep the LUN ID's the same for all igroups just for sanity's sake.
Re: Simultaneously accessing LUNs via iSCSI and FCP (different servers)
So....that makes me think I should be a bit more complete as to where I've done this -- it's been in the context of switching over ESX datastores (so was short-term use), i.e. have the same datastore mounted via FC on one host and iSCSI on another.