A Syphon server presents a single live surface, and you shouldn’t think of it as a series of frames. If, as you do here, you draw twice into the server then the second draw is likely to replace the first one before a client has seen it.
If you have to split the fields inside your app, then you’d be best drawing them in to a Syphon server side by side and splitting them again in QC – or if you can draw the entire interlaced frame then do that and perform the entire split inside QC.
That make sense?