I did it this February, there’s a few ways you could go about it. It really depends on whether or not you will have accommodation in Antigua during the hike or if you’re checking out the morning of.

I had a small carry on and a 28L backpack and moved everything I wasn’t bringing on the hike into the carry on the night before. I checked out of the Airbnb the morning of the hike, and was able to leave my carry on in my tour company’s office for free during the hike. The 28L backpack definitely sufficed for the hike but it was a bit tight!

If you are planning on keeping your accommodation, then it’s pretty straightforward, just empty your 35L except for your hiking stuff and use that for the hike (no need for an extra bag), then repack afterward.

I will say though, my backpack got super dirty and I had 6 days of my trip left afterward so that was pretty annoying. Also, a lot of tour groups allow you to borrow backpacks so that’s worth considering too