I don't think it would be too much to see two shows in one day. It just depends how far you live from NYC. If you feel like driving back from NYC after two shows is too much, then go for one. I think that's totally up to you. But I personally don't think it's too much.
I think the best way to get tickets is to use a Broadwaybox discount. You can usually get good seats for a fair price. And you can insure you'll have tickets when you get to the city. TKTS is a good way too, except you can't guarantee what it'll offer.
A little swash, a bit of buckle - you'll love it more than bread.