Hello Chris,

it looks like unsupported in the datasheet

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6021/ps6230/product_data_sheet0900aecd80246552.html

see table-2 at the line "TwinG support" it reports No

configuration guide that is shared with C4500 says:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/52sg/configuration/guide/sw_int.html#wp1128472

To use an SFP+ in an X2 port to obtain 10-Gigabit  Ethernet bandwidth, the Catalyst 4500 series switch supports OneX  Convertor modules. When you plug a OneX Converter module into an X2  port, it converts the X2 port into an SFP+ port into which you can plug  in an SFP+. An SFP+ in a OneX Convertor module provides the same  functionality as an X2 and maintains the same port numbering.

This feature is supported on Supervisor Engine 6-E  X2 ports as well as the WS-X4606-10GE (or WS-X4606-X2-E),  WS-X4908-10GE, WS-X4904-10GE, and WS-C4900M chassis.

It makes sense for a data center switch like this to support only 10GE uplinks

Hope to help

Giuseppe