It's not really possible as in this specific example. Your hosts file essentially forms part of your system's ability to map a hostname into an IP address. So when your system wants to know the IP address of "example", it'll get a single IP address in reply. The hosts file allows no way of distinguishing different "record types" for its resolution. So for hostname to IP address mapping, you only get one IP address per hostname (but multiple hostnames could resolve to the same IP address, which is the opposite).

Depending on what you're doing, might it be possible to use an external DNS service like dyndns.org to map a constant hostname, that will work on either network, to your IP address?