I've not done Tunisian crochet in a long time. Could you just work across half the stitches for however many rows you want the slit in the scarf to be and then come back and work the second half, rejoining it all when you get to the same row? It works in knitting or crochet. You'd need to hold the stitches you aren't working somehow and use a second ball of yarn or the other end of the same ball to work the second half. As I said, it's been a long time since I worked Tunisian and I may be forgetting something important.
Happiness is catching that dropped stitch while it's still fixable..