AE: Limit Characters in random range selector animation
To do this on one text layer you could use this expression in the sourceText:

Math.round(random(36));


If you'd like to keep the two layers then on the first layer use:

Math.round(random(3));


and on the second layer use:

Math.round(random(9));


The number in the brackets of the random expression is the limit, so if it's set to 100, the random expression will return a value between 0 and 100.