It seems there are plenty of PS/2 to USB converters but want we need is a composite USB input and 2x PS/2 outputs (keyboard and mouse). An example application: a server without USB ports but 2x PS/2 ports. KVM cable is a VGA and composite USB (i.e. single USB cable for keyboard/mouse). Does such product even exist?
The thing to watch for is that a lot of devices are designed to be 'converted' so those USB -> PS/2 converters which come with a lot of mice nowadays are actually dumb pin converters with the actual conversion being performed by the mouse itself. In other words, they're not converters as such and won't work with just any device.
I'm afraid I've not seen the style of device you're after, though - it'd require some decent electronics.
For example, this here:
Would rely on the keyboard switching to 'PS/2' mode.