You could add a dropdown field (call it say, display_on) to your channel with the options Domain 1, Domain 2, Domain 1 + 2.
Then in your channel entries tag:
{exp:channel:entries channel="channel_name" search:display_on="=Domain 1" site="domain_1" dynamic="no"}
or
{exp:channel:entries channel="channel_name" search:display_on="=Domain 1|Domain 2" site="domain_1" dynamic="no"}
You could also achieve this using categories instead of the dropdown depending on your content.
{exp:channel:entries channel="channel_name" category="1" site="domain_1" dynamic="no"}