I have yet to find a glue that will work with Ipe outdoors. I have not used
Epoxe but would highly recomend it if you are going to glue Ipe. Also wipe
down the joint with acetone before glueing.
I think that leaving a space between the boards may be the more successful
route to take.

gorilla glue may or may not work. part of the problem is if the wood
is not fully dry it will shrink
I think one glue that would work very well is some of the silicone
adhesive like e6000 it sticks to about anything but has a little give
to it. takes while to dry if it does not get air. but it has been one
of the best things I found to glue metal to wood or plastic.