Haven't heard of that with Sonicwall, but apparently they've broken/don't support multiple TS in same TS payload either. The config is 100% correct as generated for the proper IKEv2 usage. One of the benefits of IKEv2 is not needing multiple child SAs for such circumstances. At least for proper implementations of it.
In /usr/local/www/vpn_ipsec_phase1.php, take out this chunk of input validation:
if (($pconfig['remotegw'] && is_ipaddr($pconfig['remotegw']) && !isset($pconfig['disabled']) )) {
$t = 0;
foreach ($a_phase1 as $ph1tmp) {
if ($p1index <> $t) {
$tremotegw = $pconfig['remotegw'];
if (($ph1tmp['remote-gateway'] == $tremotegw) && !isset($ph1tmp['disabled'])) {
$input_errors[] = sprintf(gettext('The remote gateway "%1$s" is already used by phase1 "%2$s".'), $tremotegw, $ph1tmp['descr']);
}
}
$t++;
}
}
Then add two P1s with one P2 on each. That's really what you're configuring there by splitting it to two conn entries.
That validation probably isn't really necessary, might just remove that to allow configs like this. Its intention is to prevent foot shooting, but there are potential circumstances like this where it works around issues with the remote end.
