Thanks, Mark.
actually, I already had solved this problem by a not elegant way: I am using only the manual input, so I modified admin.jombib.php. I inserted, in the functions savetomysql and saveEditBib, after the code:
if($authorcount>2){
$shortauthnames = $shortauthnames." <i>et al.</i>";
}
this:
else if($authorcount==2)
{
$partesA = explode("A ",$authnames);
if($partesA[1]) { $shortauthnames = $partesA[0]."A and ".$partesA[1]; }
else { $partesB = explode("B ",$authnames); }
if($partesB[1]) { $shortauthnames = $partesB[0]."B and ".$partesB[1]; }
else { $partesC = explode("C ",$authnames); }
and so on....