Trying to hold two watermelons in one hand, are you?

Purchasing watermelon today at Whole Foods, Marash Boy remembered the caution his mother brought with her from Marash:

Bir elinde iki karpuz tutulmaz!  You can't hold two watermelons in one hand!

Of course, Marash Boy paid no attention whatsoever to the admonition, causing his mother no end of consternation.

Do you understand the meaning of the expression?