Two build engines can not build at the same time in one build definition.
2 answers
Why do you want both build engines to run? If it's for the same build, either should be OK. When you request a build, the request is picked up by one of the available build engines. When it's picked, no other build engines will see it again.
I would suggest to provide proper error messages.
Otherwise: a build request is just one request and that one will get consumed by one build and yyou can not determine the build engine.
If you want to be able to distinguish between the environments, create a new build definition for the other environment and use the build engines only for one definition.
Otherwise: a build request is just one request and that one will get consumed by one build and yyou can not determine the build engine.
If you want to be able to distinguish between the environments, create a new build definition for the other environment and use the build engines only for one definition.