The very first chest after you leave the scalding gallows and head towards drowned pass, I can see I portal pad on the ceiling, and can shoot it, I just can't find another one close enough for me to make a 2nd portal to go through.
The artifact and second portal are in the same location. In the room with the giant fan, there are 3 easily accessible exits (the way you came in, the one with the switch --look up here and see the artifact--, and the exit).
There is also a 4th exit to the giant fan room that is below the slow spinning fan. Look down as you spin around the room. It's the only recess in the wall below the fan you're standing on. You will have to float down and grapple up into the artifact room above the switch (the second portal is here as well).