There's an encounter-level-by-XP table in the DMG somewhere. It only give you values for parties of size 4, 5, and 6, but figuring this number out is trivially easy for any size group.
You have two characters, they are both level N. A level N encounter for them is two level N monsters. Two monsters because you have two characters. Any other set of creatures whose total XP also adds up to around the same value as two level N monsters would also be a level N encounter. So, a level 1 encounter for two characters is 200 total XP, as level 1 monsters are worth 100 XP and 100 + 100 = 200. A level 1 encounter for five characters (the assumed state of things in KotS) is 500 XP.
Although note that with only 2 characters, you don't have all the roles covered and the party will have serious weaknesses. I would suggest making the two characters higher level (maybe around level 5) to compensate for their numbers. You can also reduce the number of monsters in the encounters, but that won't work for all the encounters, especially the final one. You can also have each player control multiple characters, but that's a lot for them to juggle.
