Optiplex 760, splitting the monitor signal is not working

I have a problem at one of schools I am working at.

For a while we had the optiplex 755's, when it came to splitting the signal between the monitor and a projector screen it worked flawless, there was a splitter cable supplied in the box.

Then we started to recieve the Optiplex 760, these didn't come with a splitter cable so we had to source our own. The exact same ones that were used in the 755's.

Anyway we bought these splitter cables and used them on the optiplex 760 but if we had both the monitor plugged in and a projector at the sametime then nothing would display on the monitor and only on the projector, or vice versa.

The only way we are able to split a signal at the moment using an optiplex 760 is to have two splitter cables.

this is the only setup we can get working:

HDI-VGA splitter form the PC
double VGA splitter on the end of the HDI-VGA Splitter
VGA cable from monitor to one end of the double vga splitter
VGA cable from projector to other end of the double vga splitter.

It's a mess and the picture quality isn't great due to using two splitters instead of one. Anyone came across this problem?