Hello Dave,
Your windows servers are in ALB mode I suppose? In this mode, the NICs balance the TX-load and they use some multicast protocol to sense the availability of the primary adapter. If it fails, the other one takes over and as it works via multicast, the NICs can be on different switches. They only should reside in the same VLAN.
Since quite some time, HP supports port aggregation which is in fact Ether channeling. For an Etherchannel to work, NICs should be on the same device, mostly even on the same board. This is what is recommended to you. To my best knowledge, multi-device Etherchanneling is not yet possible although I have heard from experiments to loosen restrictions on this. Please check the release notes & command ref for your IOS release.
Your best option will be to check in the datasheest if the UNIX boxes support something else than port aggregation. Your vendor may be misinformed about the possibilities but as we do not know the model & type it is not possible to say something about it.
Regards,
Leo