Figure it out finally, and this is the code.

In the second window open event:

post event ue_postopen(0,0)

In the ue_postopen event:

RECT windowRect, extendedFrameBounds;
int borderWidth

GetWindowRect(Handle(this), REF windowRect)
DwmGetWindowAttribute(Handle(this),  9, REF extendedFrameBounds, 16)
borderWidth = GetSystemMetrics(5);	

x = w_main.x + w_main.width - PixelsToUnits ((windowRect.right - extendedFrameBounds.right) + (extendedFrameBounds.left - windowRect.left) + borderWidth, XPixelsToUnits!)
y = w_main.y

Declaration part:

FUNCTION long GetWindowRect(ulong hwnd, REF RECT lpRect) LIBRARY "user32.dll"
FUNCTION int GetSystemMetrics(int nIndex) LIBRARY "user32.dll";
FUNCTION long DwmGetWindowAttribute(ulong hwnd, long dwAttribute, REF RECT pvAttribute, long cbAttribute) LIBRARY "dwmapi.dll"

$PBExportHeader$rect.srs
global type rect from structure
	long		left
	long		top
	long		right
	long		bottom
end type

We have used this code successfuilly in one of our Apps:

int new_x, new_y

this.height=w_bookings.height
new_x = w_bookings.workspacewidth() + w_bookings.workspacex() + 60
new_y = w_bookings.y
this.x = new_x
this.y = new_y

return 1

W_bookings is on the left and the current window is on the right. Hope that helps.

In addition to Benjamin's excellent response, here is some additional information regarding the use of the DwmGetWindowAttribute API call:

    https://stackoverflow.com/questions/12744117/get-drop-shadow-dimensions-of-window

What will you do if the first window is positioned too close to the right edge of the physical screen to be able to see the entire contents of the second window? I don't know what your use-case for this proposed functionality is, but what you are wanting to do may not work well in all possible situations.

Best regards, John

Looks like the window size attributes in PB include the drop shadow areas that double as resize handles. You can try using DwmGetWindowAttribute to get to the actual size or hardcode the difference.