I guess you mean a cajon? Does it have a rear mic hole (good ones usually do)? I'd try that first with one mic, see if it's picking up the sound you want. If not, you may want to place the second mic directly in front, but watch for phase issues.
I normally go for a rear mic (dynamic - either a kick drum mic or a sm57 depending on the style of music/arrangement) and a front mic (either an sm57 or a SDC) to pick up more of the attack from the hands on the drum.
Your KSM's should be good for the job though, I'm slightly limited on mics sometimes as I normally record box drums with live folk/country bands and start running low on options.
Make sure you do a proper sound check rather than throwing the mics up and hoping it all works because they sound alright induvidually once you've got recording levels
The session went well. I put one mic at the back hole (about 6" away) and one across the room. It gets a nice effect. I wasn't very impressed with the actual sound of the cajon, but the set up captured the sound exactly as it is.