Well, that suggestion has merit.
At the moment you can't add the same nick twice, as you said, but you can add multiple networks to a notify entry.
So, if you use the NOTE to add descriptions like this (I used $chr(1) here)
Efnet This is Alice from Wonderland Dalnet This is Bilbo Baggins [ more ...]you could display the relevant section from an on notify event.
on ^*:notify:{
if $numtok($notify($nick).network,44) > 1 && $wildtok($notify($nick).note,* $+ $network $+ *,1,1) {
var %a = $v1
echo -stmc notify * $nick is on IRC $iif($notify($nick).addr,$+([,$v1,])) ( $+ %a $+ )
halt
}
}