Wouldn't you be able to count up to 72, not 60 with two hands?
12 from hand one + 12 * 5 from hand two = 72
12 from hand one + 12 * 5 from hand two = 72
The article made it clear, both through the text and the picture, that only one hand used the phalanx counting method, while the other hand simply used digits as we do today. Thus, with 12 phalanges to choose from on one hand and 5 digits to choose from on the other, we have 12 * 5 = 60 combinations.
Look at the example for 32 - it is given as 8 + 2x12. The hand on the right has two fingers extended. Clearly all fingers can be extended in this image, and then the number would be 8 + 5x12 = 68. If the left hand is used to count to 12, the number is 12 + 5x12, or 12+60=72.
Either the limit is 60, or the images/text in the article are wrong.
Now that I finally get what viggity was saying however, I think that what you can count to with two hands wasn't what was significant -- it was that a full, open hand was worth 60.