configurinng another interface in the same vlan would not cause an issue if stp is enabled on switch. stp mode is passthrough by default, so switch would block interface properly. anyway, when disconnecting one of interface, switch may take some time to change interface from blocking to forwarding state (stp convergence time).
sol9796: BIG-IP support for spanning tree protocols
http://support.f5.com/kb/en-us/solutions/public/9000/700/sol9796.html
sol7577: BIG-IP LTM spanning tree compatibility with tagged VLANs
http://support.f5.com/kb/en-us/solutions/public/7000/500/sol7577.html