If it doesn't have to be clientside then its easy isn't it ?

You just post back with the checkboxes, grab the data  and fill the other datgrid
with that data.

Trouble would be that you need to alter data in the select mode of datgrid, i.e.
the checkboxes. But it can be done using control collection of datgrid within
the Item_Select ( I think!)

foreach (Control c in Datagrid1.Controls[x].Controls)
{
Response.Write(c.GetType());
}

where x starts at 1 and and at number if items in grid-1.

or use an if control == checkbox

or use loop loop.

Bit of messing should sort it.

Or use edit item to ... but that requires a postback each time  you check a box.