There is no support for SSS in simplewallet. This would require some amount of new modexp code to be added (unless a variant of it can be made using EC cryptography, but I'm not aware of any).
So the best you can do is use a standalone SSS tool such as
ssss (see http://point-at-infinity.org/ssss/). It may already be in your distribution's packages.
Example, with the words "shamir secret sharing":
$ ssss-split -t 2 -n 3
Generating shares using a (2,3) scheme with dynamic security level.
Enter the secret, at most 128 ASCII characters: Using a 168 bit security level.
$ ssss-combine -t 2
Enter 2 shares separated by newlines:
Share [1/2]: 2-6191ac079ef7624a72ffce0da2372d6a24fe2e530c
Share [2/2]: 3-d2045f94bf624c6d1e52ca3d14a3f512ffd8c19459
Resulting secret: shamir secret sharing
For more info,