Can two DCs share one GDI object (brush, pen, etc.)?
Can two device contexts (DCs) share one GDI object, such as a brush or a pen?
The documentation for SelectObject mentions that Bitmaps cannot be selected into more than one DC at a time.
Essentially this is because GDI operations can write to a bitmap, and it would be complex to manage multiple write operations at a time. However all the other GDI objects, once created (with the exception of regions - but those are always copied), are immutable, therefore have no restrictions on being selected into multiple DCs at a time.