I got a temporary solution to my problem by making main2.com and addon domain instead of a parked domain and doing a .htaccess redirect to main1.com (I used addon instead of parked because while creating a redirect, the existing WP3 .htaccess of the main website is edited and sometimes corrupted, so the the new redirected .htaccess resides on a different folder from the main website). However, I am not very good at editing .htaccess, therefore I am not able to make a complete redirect of e.g. main2.com/members/chgpass.php to a corresponding main1.com/members/chgpass.php