We have brought up our children with two languages, General American English and Taiwan standard Mandarin Chinese. The second of those is not a NATIVE language for either parent, but we both speak it. We have lived overseas during the lifetime of our three older children, and at one time my two oldest sons preferred speaking Mandarin to each other to speaking English.

It's very helpful for parents attempting to maintain bilingualism to find lots of children's books in the less encountered language, and also listen to Internet radio and otherwise keep exposed to the language.