Hello Alex,
it will never work well
even if you could install two NM GE on the C3825, the C3825 works in software and it not able to handle the load of even one single GE.
one GE full rate means 1480000 pps per direction (smallest size 64 bytes frames)
A 3825 is able to handle 350000 pps with CEF.
Also port channels on routers are not a good idea.
Other colleagues have reported problems in attempting to use two 2GE bundles on a C7201 with 4 GE ports.
see the attached file for performances with raw data (no features applied)
Use two routed L3 links instead of port-channel. Use subinterfaces on the existing GE ports it is a reasonable solution.
Hope to help
Giuseppe