The effects potentiate each other, so you will be tripping much harder.
If you tolerate weed well otherwise, it can work out well. If you’re prone to paranoia, anxiety, and similar effects, this combination can great increase them.
Now that I've experimented with the combo, I would say that cannabis interferes with the profound impartiality of ayahuasca. I enjoy ayahuasca more without cannabis.
"The effects potentiate each other, so you will be tripping much harder." -- This is not how I experienced it. To me, the effects of each substance are pretty orthogonal. The combination is not multiplicative or even additive. It's more like being on two separate substances that affect different aspects of experience. However, the cannabis made me less able to adapt as the force of the ayahuasca got stronger.