If you don't want to change lens, there's no choice but to have a 2nd body or else compromise your whole shoot with your all-in-one.
I would not suggest doing that as you are just the backup photographer. During the day, under different situations/locations, you would know which of your existing lens to mount on your D50.
Personally, I find that there's no point in purchasing the D70S or even the forthcoming D80 based on your current setup.
I don't even consider myself an "average" yet but I still make do with a basic DSLR (the D70S was the most basic when I purchased mine) but saved the money for good glass. It's good glass over body for me; esp. with digital.
I have so far managed one as a main + one as backup on a D70S + 18-70mm + 17-35mm + SB600. All the above are rather basic gear.
The experience of shooting a wedding and the planning before the shoot is what you should be thinking about now.