No multiple parents are not supported. The parent/child relationship has been designed for container/contained relationship. Example: the web app (=child / contained) needs to be deployed in tomcat (= parent / container). From a logical point of view, the web app cannot leave on its own: it needs a container to be deployed in => hence the tight parent/child relationship.
There is no parent/child relationship between a web app and mongoDB. I believe you are thinking more in terms of dependency, as in your web app requires the DB to run. glu does not support expressing a dependency like this. This has been a design choice as your application should not depend on the DB to always be up: there will be times when the web app will be up but the DB will be down or vice versa (which cannot happen in the case tomcat / web app: the web app cannot be up if tomcat is not up...). So your app needs to handle the cases when the db is up or down properly.
I do understand the requirements, but I still believe your app should be able to handle up/down at the db level as one day or another it will happen.
Yan