Hello,
I think you need to configure stacking for both modules, here is what CCO says:
Configuring Stacking
Stacking is the connection of two switch modules resident in the same chassis so that they behave as a single switch. When a chassis is populated with two switch modules, the user must configure both of them to operate in stacked mode. This is done by selecting one port from each switch module and configuring it to be a stacking partner. The user must then connect with a cable the stacking partners from each switch module to physically stack the switch modules. Any one port in a switch module can be designated as the stacking partner for that switch module.
Beginning in privileged EXEC mode, follow these steps to configure a pair of ports on two different switch modules as stacking partners.
SUMMARY STEPS
1. interface fastethernet interface-id
2. no shutdown
3. switchport stacking-partner interface FastEthernet partner-interface-id
4. exit
5. interface fastethernet partner-interface-id
6. no shutdown
7. end
HTH,
GP