RSPAN.
Check out this link:
http://www.cisco.com/univercd/cc/td/doc/product/core/cis7600/software/122sx/swcg/span.htm#wp1059942
The 6500 IOS is similar to the 7600 IOS in native mode (you are referring to Native mode, aren't you? :^) ).
Basically, on the one 6500 that you are sniffing from, your config would look like this:
vlan 500
remote-span
monitor session 1 source int (server's int #1)
monitor session 1 source remote vlan 500
monitor session 1 destination (sniffer int)
On the other 6500:
vlan 500
remote-span
monitor session 1 source int (server's int #2)
monitor session 1 destination remote vlan 500
I just picked 500 for the example. Don't forget to allow the remote vlan across your trunk link, preferably (if you are expecting a lot a traffic), across a trunk link that does not have a lot of traffic across it.
Hope this helps.
Eugene