im in the dark brotherhood and thieves guild does that count
The only real requirement, that I can remember at least, is related to fame/infamy. I'm thinking you need <= 1 infamy in order to do certain stage(s) of the quests involving some shrines, but it's been a while since I did the quest line, so I may be a little off on that. Also, if I'm not mistaken you'll become unable to equip any of the crusader equipment if your infamy ever exceeds your fame later in the game (though getting your fame higher will allow you to use the items again).
If you've been doing the Dark Brotherhood quests, I'd expect that your infamy might be fairly high already, so you may need to work on doing certain quests and things that raise your fame up. And as TheUnadoringFan said, it may not make sense from a role playing perspective to have one character that pursues both quest lines; unless perhaps he/she does one or the other first, and later on has a change of heart or some such.