You can adapt the following code as you need, but this gives the principles of what you want to do. Copy the following into a module:
Rem put this code in a module so it can be used by any form
Public Sub alignForms(pFormLeft As String, pFormRight As String)
Rem check that 'left' form is loaded
If SysCmd(acSysCmdGetObjectState, acForm, pFormLeft) = 0 Then Exit Sub
Rem check that the form is not in design view
If Forms(pFormLeft).CurrentView = 0 Then Exit Sub
Rem now calculate the required position of the right form relative to the left form
Dim lngTop As Long, lngLeft As Long
lngTop = Forms(pFormLeft).WindowTop
lngLeft = Forms(pFormLeft).WindowLeft + Forms(pFormLeft).WindowWidth
Rem use DoCmd.MoveSize to position the right form
DoCmd.MoveSize right:=lngLeft, Down:=lngTop
End Sub
... and copy this code into your form's Open event handleer:
Rem put this code in the form to be aligned with an open form
Rem the form you want to align with is identified by its name ("TargetForm" in this example)
Private Sub Form_Load()
alignForms "TargetForm", Me.Name
End Sub