This worked in a test project ...
if "$(ConfigurationName)"=="Debug" copy "$(ProjectDir)Package.appxmanifest" "$(ProjectDir)Package2.appxmainfest"
The only real difference is I added quotes around Debug. It does not copy the file without the quotes. According to MSDN, you need to separate commands by line breaks. Yours should probably look something like ...
if "$(ConfigurationName)"=="ReleaseOEM" copy "$(ProjectDir)PackageOEM.appxmainfest" "$(ProjectDir)Package.appxmainfest"
if "$(ConfigurationName)"=="ReleaseOEM" copy "$(ProjectDir)StoreManifestOEM.xml" "$(ProjectDir)StoreManifest.xml"