Do you have the exterior ramped GC column on your EA IsoLink? If so, it'll be difficult to fit a longer GC column inside the shallow compartment. If your column is located in the internal compartment, there's more space for a longer GC column, which might help separate the N2 and CO peaks more effectively.
Hopefully someone will offer a simple solution, but on the extreme end you could also consider additional capillary GC separation like Paul Brooks' suggests in the 2010 Isogeochem post linked here: https://list.uvm.edu/cgi-bin/wa?A2=ind1007&L=ISOGEOCHEM&P=58792 (July 2010, post ID = 58792). I've thought about implementing something similar on our TC/EA for organic samples, but haven't worked up the nerve yet.