Using different vendor's (for sake of simplicity let's refer VMware as a vendor for SW iSCSI initiator) initiators to access the same LUN is not supported. You'll have problems with path failover.
AFAIK it is supported to have different initiators (i.e. qlogic and emulex iSCSI HBAs) on the same host as long as they're not used to access the same LUN(s).
Different HBA models from the same manufacturer should be OK since they normally talk to each other in "same language" however this should be checked with the vendor.
WBR
Imants