In the location of the stabbing, it is the same location where you would do a chest decompression. I would put on an occlusive dressing (BLS can treat a sucking chest wound and "burp" the dressing)
The GSW can present with little to no symptoms, aside from a hole in the belly, and the patient may look fine. As we know, the abdominal cavity will fill up and pool A LOT of blood. The patient will be going into shock and we will hardly notice it because there is little blood loss.
As for the stabbing, the location isn't quite as bad, but still pretty serious. There are lots of vessels on the inferior/posterior of those ribs. Having them hit with the knife can lead to serious problems. This patient will need a chest tube most likely, which can't be done in the field.
Like Herbie mentioned, these situations are just that... situational. Keeping calm and looking at the big picture is what is important here (if we are talking BLS or single crew). Using your best judgement and following triage protocols will most likely help you. It is hard to say which patient is more critical without more information.
If it were me, a single crew. I would do the rapid trauma assessment, stabilize best I can, and transport both at the same time and request another unit to rendezvous en route if possible.