I'm not so sure that singlecard/multicard mode has anything to do with STP on E-Series cards. From the book Optical Network Design and Implementation:
"It is important to remember that in the case of the E-Series cards, the Timing and Control 2 (TCC2) common card actually runs the spanning-tree engine with a single instance of STP per node. Each E-Series card plugged into the chassis maintains only a copy of its Layer 2 forwarding table. In the case of ML-Series cards, however, the individual cards maintain copies of the Layer 2 forwarding table and they run an instance of STP." ... "From an STP perspective, however, the entire ONS node is perceived as a single bridge, even though it might contain multiple E-Series cards. For ML-Series cards, however, each card is accounted for as a discrete bridge, and STP uses each and every ML-Series card for its STP computation."
