My friend almost bought a trailer like that. The reason they didn't connect the two was pure laziness. My own trailer is a common rear-kitchen layout and I've found that lower end models of the same floor plan don't connect the kitchen's grey tank to the forward dump connection. But better built units do indeed connect them - as they did with mine.
At any rate I think it would be a huge inconvenience to have to dump from two spots 10 or more feet apart and I would plumb the grey tank to the dump fitting if it were mine.