Yes, nmt would form a fumerate, giving you a mixture.
The simplest method of separation is fractional crystallisation, as kloudQ7 has suggested.
This doesn't always work though as the solubility of nmt in aliphatic hydrocarbons is poorer than dmt, meaning that nmt should drop out of solution first.
If you are looking for a chemical means of separation, then two methods have been discussed here,
Acetylation with aspirin or carbamic acid formation with carbon dioxide.
Acetylation is more likely to be successful, I suspect.