How can I ‘join’ two metrics in a Prometheus query?

You can use the argument list of group_left to include extra labels from the right operand (parentheses and indents for clarity):

(
  max(consul_health_service_status{status="critical"}) 
  by (service_name,status,node) == 1
)
   + on(service_name,node) group_left(env)
(
   0 * consul_service_tags
)

The important part here is the operation + on(service_name,node) group_left(env):

• the + is “abused” as a join operator (fine since 0 * consul_service_tags always has the value 0)
• group_left(env) is the modifier that includes the extra label env from the right (consul_service_tags)