Tandem nursing is much easier than nursing while you're pregnant; if you make it through the pregnancy, you've definately made it through the hard part.
I'd advise you to take a vitamin. I find that on the days I do I feel much more like a human being and I can even sleep better. I guess with three people making demands on my body, my vitamin deficiencies become more apparent.
I know what you mean about your physical strength flagging, and a vitamin can definately help with that. I don't have any recommendations for organic, wholesome supplements, because I'm poor, but I take generic walmart multivitamins and even those help me a lot.
I've got a six month old and a two year old who are both nursing happily, and as long as I get my vitamin all is good.