Hi,

I think one of them is for switch processor and the other is for route processor as mentioned in this link http://www.cisco.com/en/US/products/hw/modules/ps2797/products_data_sheet09186a0080159856.html

• WS-SUP720 supports 512MB of DRAM for the route processor and 512 MB for the switch processor

• WS-SUP720-3B supports 512MB of DRAM for the route processor and 512 MB for the switch processor, upgradeable to 1GB

• WS-SUP720-3BXL supports 1GB DRAM for route the processor and 1GB DRAM for the switch processor

Therefore you need two 1GB DRAM if your model is WS-SUP720-3BXL

Regards,

Dandy