I couldn't find this anywhere, but here we go.
dynamips.exe -P 2600 -r 128 -X -t 2611XM C2600-AD.BIN -p 1:NM-4E -p 0:16:WIC-2T -p 0:32:WIC-2T
By default the 2611 has a net module in slot 0, a dual port FE (CISCO2600-MB-2FE) so you can't change that. But you can put a net module in slot 1. WIC slots map weird. Also the 2611 is stuck to only 2 WICs.