just wondering if anyone has ever thought about spraying/painting there locos, coaches, wagons etc a different livery either side. i think it would work...as you rarely see both sides of a model on a layout, specially a end-to-end one. i know it wouldnt work on some, but surely would be ok for 90% of models out there.

It wouldn't work on my current layout, but my next one will fill a room, so that idea would work.  I think this would mainly be a good idea for end to end layouts (like you said), or owners who don't have the room for a lot of different stock.  Good idea though.

I recently did two three-car thumper units with Electra overlays, one blue and one blue-grey. With hindsight, i wish I'd done all six cars blue one side and blue-grey the other. On a u-shaped layout, there is no danger of ever seeing the other side.

Problems might arise when/if you ever wish to sell the loco.

You could always obtain a spare body rather than mix-matching one and just swap the bodies over when you feel like a change. Your idea would work on my layout as the rear half cannot be seen but it wouldn't work from a practical point of view as my locos only tow one end as the other is super-detailed.