Hi
If A.exe also a .Net exe then you can add reference the A.exe to B.exe. The A.exe is in its own namespace.
Then set the B.exe's form's
isMdiContainer
property to true. Now you can open A.exe's form1 as child here
private void Form1_Load(object sender, EventArgs e)
{
Form frm = new A.Form1();
frm.MdiParent = this;
frm.Show();
}