Mine was split into two due to weather; it turned out to be worse than forecast and we found out when we reached the first SHP for the Nav.
It caused me some problems with my general handling - I was very nervous and had been looking forward to the less intensive period of the Nav to settle down, but then again I think I would have been very weary in the circuit if I had done it all in one go.
Ultimately it's down to your own preference, but you shouldn't waste a good weather opportunity to do the Nav - General handling is much easier to slot in.