You can have multiple Switched Port Analyzer (SPAN) sessions configured at the same time. But there are certain restrictions while configuring additional SPAN sessions.
For more information on SPAN and Remote SPAN (RSPAN) session limits, refer to these documents:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a00801a5b33.html
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a00801a5b33.html#1033304
For multiple SPAN sessions, add the create keyword at the end when configuring the additional session, as shown:
For configuring the first SPAN session:
6509> (enable) set span 3/1 2/1
For configuring the additional SPAN session:
6509> (enable) set span 3/5 3/10 create