The multi-vlan feature was originally supported on the early 2900XL hardware since the asics used at the time DID NOT support ISL or dot1q encapsulation. When the new 2900XL hardware and the 3500XL were released they had asics that DID support ISL and dot1q encapsulation (due to customer demand) and so multi-vlan support was no longer required. A decision was made to keep the multi-vlan feature available for these platforms... I assume... so that customers with a mix of older XL and newer XL hardware could configure them the same. Now, with information above there is really no need to configure both on the same interface as multi-vlan was a band-aid for non-trunk support. Logically, does not make sense to configure both features in the same interface but should be okay to configure on the same switch on two or more different interfaces assuming that the XL's have the newer ASIC that will support both multi-vlan and trunking..
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