According to the documentation (http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver), it's been possible to define multiple resolvers since 1.1.7.

However the documentation does not provide the syntax for doing so. Separating the IP addresses with spaces seems to work for me (in 1.4.2):

resolver 10.0.0.254 10.1.0.254 8.8.8.8;