Indoor Outdoor Cat won't poo in litterbox

Wasn't really sure where to post this question, or if there's really anything I can do about it, but just wanted to see...

I have 2 indoor/outdoor kitties, (they are about 2) and have had them just over a year. I luckily live in a place where rain/inclement weather is not an issue. However, it has rained here for about a week straight. Since then, I have been keeping them inside quite a bit. They don't like it, but they'll both go pee in the litterbox, but neither will poo. I try to let them both out at least twice a day even if it is really brief (and I have to dry them off and wipe their muddy feet.) but I can't really let them run free. One of them is awful, I swear he rolls around in the mud and puddles and then comes in and cries at me that he is a mess!

They usually go to the bathroom right after breakfast and dinner but I feel like now they are all mixed up, and possibly holding it in. Maybe not even going? I thought about this the other night when my boyfriend picked one of them up, and he farted bigtime. Poor little buddy. This morning, he threw up undigested food too.

What can I do though?

Empty a litter box. Go outside and fill the litter box with dirt and see if they'll use that. If they do, give them a few days, and start adding kitty litter to the dirt until they're weaned.

@Chromium Blues  That is a fantastic idea!

However, since it has been raining for so many days, all I have to offer them is mud, and that kinda defeats the purpose. (Also, one of them likes to o in bushes only I think. Anyway, I broke down this morning after breakfast when one of them peed in litterbox... well, sort of... he didn't even squat, just stepped in box and peed, getting the wall instead. Not cool. So, I decided they can go out after meals even if it's raining, and I'll just have to deal with it.

Thanks for your suggestion.