Are methyl n-propyl ether and methyl iso-propyl ether metamers?
The term metamer is equivalent to isomer.  Isomers differ in connectivites of atoms between molecules with the identical number and type of atoms.

Methyl n-propoyl Ether and methyl iso-propyl ether have the same numbers of carbon, oxygen and hydrogen atoms.

As an aside, I have never heard the term metamer before, I expect it is archaic.