I think the main reason for the 2piece shaft is to prevent it from deflecting enough to fall out of the transmission. The longer the driveshaft, the more the pipe can wobble out of round under severe loads. The whole trick with the 2piece shaft is to keep the transmission end perfectly straight so that you don't have friction in the front ujoints. That way the driveshaft is nothing more than an extension of the transmission output shaft and you don't loose much power turning it.

I think you could get away with a small diameter front half or one made from aluminum or carbon fiber if you wanted to go exotic.