Sending tracking requests to multiple Matomo servers is easily done using the Matomo JavaScript tracker.

The standard Matomo JavaScript tracking code contains the following lines for tracking to a single Matomo server:

_paq.push(['setTrackerUrl', u+'matomo.php']);
_paq.push(['setSiteId', YOUR_SITE_ID_HERE]); 

To add a second tracking URL to Matomo, we can add the following code below it:

var secondaryTracker = '';
var secondaryWebsiteId = YOUR_SITE_ID_HERE;

// Also send all of the tracking data to the new Matomo server
_paq.push(['addTracker', secondaryTracker, secondaryWebsiteId]);

It is not necessary to have the same Site ID in both the Matomo servers (Each tracker can have a unique Site ID for the website you are tracking)

Adding additional Matomo servers can likewise be added with the following:

var tertiaryTracker = '';
var tertiaryWebsiteId = YOUR_SITE_ID_HERE;
_paq.push(['addTracker', tertiaryTracker, tertiaryWebsiteId]);

There is no limit to the number of trackers you can add via this method to send tracking requests to multiple Matomo servers.