If you put both requested trips on line #1, you will get the swap only if you can be awarded both of them. If all the trips cover the same footprint it shouldn't be a problem unless the two requested trips don't have legal rest time between them. I'm not sure what that is and I'm too lazy to look it up...
If you put the two requested trips on different lines, you're telling PBS you want the trip on line #1 as first priority OR the second trip on line #2 as second priority; you won't get both that way.
In the first case, if the footprints of the trip you're trying to drop and the ones you're trying to pick up don't match, reserve coverage will come into play.