For purposes of standby, EWR and LGA are not co-terminal airports, and you therefore cannot switch. LGA and JFK are considered co-terminal airports. The only time CO considers EWR and LGA co-terminal airports is at their convenience (e.g. MX problem cancels your LGA flight, and they have seats to EWR).

For more elaboration including CO Insiders comments, you can search for a thread I started in July on "stand-by from co-terminal airport"

I have not yet figured out how to embed a link inside my posts!