As far as I know RadGrid does not support multiple insert forms visible
simultaneously. One suggestion to achieve multiple insert is by using an external form with multiple
fields for insert. Thus when pressing a button from that external form
you can extract the values from the input fields, insert the new
records in the grid source and rebind the control to reflect the
changes.
You can go through the following online demo which demonstrates how to
insert several new records in the grid with a single button click using
a ToolTip. Multiple Insert in RadGrid using ToolTip