Different master page header on the first page of report

I need to have one master page header on the first page and another master page header on the next pages of report.
I am using 3.7.1 version.

1. I inserted a grid with two rows to the master page header.
2. I put one new grid into each of the rows.
First nested grid for the first page header, second nested grid for the next pages header.
3. I set an onRender scripts:

//first nested grid
if( pageNumber > 1 ){
this.getStyle().display = "none";
}

//second nested grid
if( pageNumber == 1 ){
this.getStyle().display = "none";
}else{
this.getStyle().display = "block";
}

This works ok with the sample database, from JDBC Data Source.
But I need XML Data Source and with that it does not work at all.
It seems as if with the sample database table, the onRender event gets called on every page, but with my XML Data Source it only gets called once.

Please, If you could help me solve this mysterious problem I would appreciated it very much.

erisa