Several Carnival ships have a few category 1A (lowest cagetory interior) cabins which actually have two portholes. The cabins have one twin bed and a sofa. I discovered the cabins a few years ago while browsing at the stateroom symbol legend in Carnival's brochure. Those cabins are great for me, since I travel solo. I get an outside cabin for the inside rate. I also like their location, which is at the front of the ship (quiet). I have stayed in porthole cabins on the following ships - the staterooms are as follows:
DESTINY - cabins 1101, 1102, 1105, 1106, 2101 2102, 2104, 2106
TRIUMPH, VICTORY - cabins 1201, 1202, 1205, 1206, 2211, 2212, 2216, 2217
Check the brochure for cabin number accuracy - in case I made a typo.