Is there a function to separate strings?

You can use Lang.String.substring(startIndex, endIndex), where startIndex is included and endIndex is excluded, both are zero based.
So:

var two = strSec.substring(0,1);
var three = strSec.substring(1,2);


I don't think you should use this to split numbers though. See next example:

var sec = 23;
var two = sec / 10;
var three = sec % 10;


And then you can use format function to make a string if you want.