Craft 3 live previews across different root (or sub) domains

.env.php

<?php
// In general.php define an alias:

'aliases' => [
    '@baseUrl' => getenv('CRAFTENV_BASE_URL')
],

// In the CP, set the Base URL for each site to @baseUrl/
// When previewing, the CP will therefore always use the hostname the editor logged into as the base url for the preview
// (that happens with 'new' entries anyway regardless of base url, but existing entries use base url).
// We'll switch Craft to the appropriate site for the live preview request, but without changing the domain.
// So no need to worry about the cookie domain or CORS. 

$craft_site = '';
$httpHost = '';

// always make sure your sites only load at known domains
// see: https://expressionengine.com/blog/http-host-and-server-name-security-issues
switch ($_SERVER['HTTP_HOST']) {

    case 'red-pill.com' : 
        $httpHost = $_SERVER['HTTP_HOST'];
        $craft_site = 'redpill';
        break;

    case 'blue-pill.com':
        $httpHost = $_SERVER['HTTP_HOST'];
        $craft_site = 'bluepill';
        break;
}

// is an editor live previewing a site other than the one they logged in to?
if ( isset($_POST['livePreview']) 
    && $_POST['livePreview'] === 'true' 
    && isset($_POST['action']) 
    && isset($_POST['siteId'])
    && isset($_SERVER['REQUEST_METHOD']) 
    && $_SERVER['REQUEST_METHOD'] === 'POST') {

    switch ( $_POST['siteId']) {
        case '1':
            $craft_site = 'redpill';
            break;

        case '2':
            $craft_site = 'bluepill';
            break;
    }
}

if ( ! empty($craft_site)) {
     DEFINE('CRAFT_SITE', $craft_site);
}

@bwing with the above you can sign in to either domain and view previews for the other site(s) without actually changing the domain. Essentially we are telling Craft to load site 2 at the domain for site 1 (or vice versa). So you only need to be logged in to one site to preview any other site.