You can assign the file path you got from the OFD to a string variable and then use that variable with button2.
edit: VS is taking foreverr to load, so this is freehand:
vb.net Code:
Public Class Form1
Dim filepath As String
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
OpenFileDialog1.ShowDialog()
filepath = OpenFileDialog1.FileName
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
MessageBox.Show(filepath)
End Sub
End Class