What the POOP do we do?

     Johnny and Shaine have been potty trained for 2 months now, but they still wear  pull-ups at night when they sleep.  Shaine will nap without a pull-up on and is doing great staying dry,but now we're having some poopie issues.  Johnny has refused to poopie in the potty.  He will ask for a pull-up to poop in, and Shaine is starting to copy him.  Mike and I have tried so many different attempts to encourage them to use the potty.  Stickers, candy, reading stories, and signing songs.  Oh my goodness...I made up a song last night for Shaine called Princess Poopie.  It was about a poopie that wanted to come out and go swimming in the potty before it gets too late.  Ha ha ha, as gross as that may sound I was pretty impressed with my singing and rhyming talents about poo.  I had asked their doctor about what we can do with the poop issue.  I was given some information on Toilet Resistance training that tells us not to talk about using the potty.  Don't push the kids to use the potty and keep the pull ups off of them as much as possible.  In fact, it says not to talk about it at all and let them figure it out on their own.  Well, guess what happen?  They both held their poop for 2 days getting constipated and stomach sick.  Now what????   I don't want them to hurt or get sick and we are definitely tired of spending tons of money on pull ups which is just as expensive as diapers.  Here's the thing...they will always pee in the potty, but they just refuse to poop without a diaper or pull up.  Oh my.  If anyone has any helpful hints or suggestions please send them our way.  I so tired of poop!