I'm currently tandem nursing my 6-wk-old and my 2 yo.
I think most women find that nursing through pregnancy is the hard part, and tandem nursing the good/easy part. For me (so far, anyway), it has been the opposite. Nursing while pg was not too hard on me. Some nipple pain in the first trimester, but that went away fairly quickly and never bothered me too much.
Be prepared for an increase in nursing when your milk first starts to dry up. When this happened (around 15 weeks or so), DD was nursing for literally an hour or more at bedtime. It was really painful for me and not at all good for our nursing relationship. DH and I found that introducing a bottle of cow's milk at bedtime helped to smooth this transition -- she got the milk she needed and some of the soothing she needed from sucking, then she'd comfort-nurse for a few minutes before falling asleep.
We also nightweaned around this time, which helped improve my sleep slightly, but not entirely. I would HIGHLY recommend that while you are nightweaning (if you plan to do so), you should also transition to your DH being your toddler's primary nighttime caregiver. I wish with all my heart that we had done that before the baby arrived. By the time I was in my 3rd trimester, I was exhausted and resentful of everyone in my family, every night, because I could. not. sleep. DD still wakes 1-2x a night and will go into hysterics if DH tries to calm her. It was really terrible for a few months; I was not a pleasant person to be around. I would have done this differently.
Nursing in labor was actually super helpful when my labor stalled. The L&D nurses were delighted that we brought DD along for our birth.
Once the baby arrived, things actually got much harder, IME. DD (who had been nursing 3-4 times a day and never at night) suddenly started begging to nurse obsessively. All night, all day. Every time my NB nurses, she melts down: "NO baby mama milk! MY mama milk!" It has been a major point of conflict since DS was born. At the same time, I'm glad she's still nursing, b/c she's still such a baby and needs that closeness and reassurance from me. Hard as it's been, I don't regret making the choice to tandem nurse. We're still trying to navigate these waters -- setting limits on the toddler's nursing, finding ways to distract her, trying to RE-nightwean her, etc. And the good thing is, with all these nurslings, I have a crazy huge milk supply -- much more than I did when DD was a NB.