Of course on the engine side you have double the cost , but the C20 is so reliable that really should not be an issue . There are now so many being parted out the price of spares is v low . If you were buying one to run for a while i would say buying a second one for parts would make sense if you can find one for around £100k - £150k . Then you have a spare of everything and are still all in for around £500k !!!
Also dont forget that you will use over 20% less time per mile !!