How to Successfully Implement Identity Merging Across Different Domains and Platforms Using Mixpanel

Hey everyone! We've been trying to get identity merging (we're using simplified merge, not original) to work for a user who goes from Website A to Website B (on totally different domains), but with no luck. Every iteration the profiles don't get merged On Website A we do the following logic:

var did = mixpanel.get_distinct_id();
mixpanel.identify(did);
// code to append `did` url param as `mp_id` to Website B's page that is then opened via a button click

On Website B we do the following logic:

const mpId = urlParams.get('mp_id');
mixpanel.identify(mpId);

we've tried variations where we set the $device_id and $user_id properties explicitly on Website B so that it would directly match with Website A, but that also didn't work How do we get identity merging to work across two totally different domains? We also need a solution that works from web -> native (we use appsflyer to handle the deeplinks, and we do very similar logic where we read in a url param and call identify)