From the internet - not my suggestion...
In the first window, put the current record's primary key into a global field. Base a self-join relationship on this global field. In the second window use a single-row portal to the self-join to display the related record (which will always be the same record that's in the first window, though it might need a refresh to display). Use a onRecordLoad script trigger in the first window to update the global field when the record changes.
The portal in the second window probably won't be needed, just put the related fields on the layout.
Home > Using FileMaker Pro > FileMaker Pro basics > Managing files > Opening multiple windows per file
You can open more than one window per file. This allows you to view the same layout in separate windows, or to simultaneously view different layouts in the same file. You can perform sort operations and find operations independently in the different windows.
" I have created a button on the first layout that that will open a second layout next the first."
Why not avoid the problem and just make a single layout that looks the same as the two layouts you have now side-by-side?
Ah! - because your first layout is in list view?
No, both open initially in Form View.
Its becasue the second layout can be resized and moved around the screen. Its postiion and size recorded and it will open again the same.