Is it possible to consolidate these two pages into one? A single page is capable of obtaining the information you are after.
If it is not, you could use the customization editor in advanced mode, which allows you to edit the HTML page directly. The following block from logon.inc allows for the values of the field to be pre-populated if the value already exists. You could modify this if statement, in the logon.inc file for your second login page to permit the username field to be pre-populated.
//Enable auto-population only for read-only text field
if ( ("1" != $fields_settings[$i]["rw"]) && ( "checkbox" == $fields_settings[$i]["type"] || "text" == $fields_settings[$i]["type"] ) ) {
$fields_settings[$i]["value"] = '%{session.logon.last.'.$fields_settings[$i]["varname"].'}';
} else {
$fields_settings[$i]["value"] = '';
}
Change the first line to the following. I've added an additional OR statement that will equate to true if the current field is the username field.
if ( ("1" != $fields_settings[$i]["rw"]) && ( "checkbox" == $fields_settings[$i]["type"] || "text" == $fields_settings[$i]["type"] ) || ( "username" == $fields_settings[$i]["name"] ) ) {
I've tested it a bit on my box and it seems to work well, and permits you to change the second username if needed.
Hope this helps!