I had drawn that diagram in my last post based on the connection of a USB y cable , my issue is not with the wire or how to connect it , it is with safety of the 5V going out of the port when it is set to "USB host mode(with charging)" and that 5V is supplying the same current like in boost mode(measured with a 500Ohms load resistor) ,
just as Pail said
this means the port starts with outputting 5V on the port, so with power connected to the port, the 5V-1200mA from the charger may not always be exactly the same as the 5V going out of the port and that will produce voltage difference, anyone who made battery packs knows how much current can come out of placing 2 cells with slightly different voltage in parallel,
anyway my concerns are with long term use, i don't think it would blow up the 2A fuse on the V+ of the n900 USB port, so have anyone tried it yet? i'd like to know who actually got or built a Y cable and is using this for a while with no issues.

also what happens when the power lines while in boost mode get shorted, say a bad USB flash drive with a broken shorted out connector was connected to the device, what will happen?
in a normal USB host like with a laptop if the USB power got shorted the USB controller cuts the power and in some computers the system will get a notification of the short