Unless I'm misunderstanding your question, I'm not actually using any virtual ports from Serial Splitter. I'm just redirecting data from a usb to serial port to an actual physical (PCI) serial port.
When I first open the port in Serial Terminal, I get a portion of the end of one packet, and a full packet following it all at once. (Likely, this is the amount of the data that's still buffered in the usb to serial chip that was waiting to be read.) After that, the packets seem to come in once at a time in the proper order.
This is a screenshot from after I cleared the edit view: http://i.imgur.com/JWcUBBu.png
Here's an animated image of the data coming in: http://i.imgur.com/FAt6bRr.gifv You can see each packet starts with the ".Ɖ", (the hex 0x02D0 that is my STX)
Here's another animated image of me transitioning from Serial Terminal to Serial Port Monitor: http://i.imgur.com/OTjk9at.gifv -- You can see the data coming in as expected in SMP -- the 0x02D0 leading the packet.
And here's Serial Splitter in action: http://i.imgur.com/WTJ62ZN.gifv
In this image, I add the redirect, and then click back over to Serial Port monitor, and you can see that each packet it sees there is starting with 0x3800 rather than the 0x02D0, as in the screenshots before.
It definitely seems like the Serial Splitter is somehow reading differently from the port than the Serial Terminal does.
Is there anything else I can try and get you an image/gif of?