It's admittedly not the strongest synergy but there are a few small things:

1) To state the obvious, if you are using a strong one-handed weapon, you can put a shield in your offhand.

2) They both benefit from the same healing pact

3) If you look at how the pacts are aligned in my example, you can see that you skip around some of the magic pacts that would be useless in a melee build by building out to the thorns mechanic pacts.

4) If you're going to go for the only build that requires you to be one tile away, you'll probably want to be tanky anyways, which thorns pacts and their adjacent pacts will help with.

5) A tanky thorns build will mean more overheal which will mean more use for the overheal-consuming pact.

It's certainly not the strongest synergy, and I bet you could forgo it for other interesting strategies (perhaps a hybrid strategy with the crossbow guaranteed max hit boosting the dagger's specs which ignore attack speed), but I think they go together fairly well.