Not have two beans, brain cells, etc. to rub together In english explanation

This idiom's been used to indicate the halfpenny since the mid to later 1800’s. Maybe the idiom refered to rubbing to halfpennies together to create one full penny. However, rubbing coins together, historically, had a more practical, if dishonest, purpose.

Although it is hard to know how rubbing coins together is related to being without money, it may have its origin in the debasement or “sweating” of coins. When coins were made with gold or silver, which were both quite soft, it was possible for a person to remove bits of precious metal by rubbing the coins together. This was usually done by placing the coins together in a bag and shaking them, which allowed the friction between the coins to remove fine gold or silver particles or dust from the coins, which would collect in the bottom of the bag.

The coins themselves would appear to be naturally worn and could be circulated at face value, allowing the cheater to collect the gold or silver dust until enough was gathered to be melted into bullion and sold at market value for a handy profit.

Even the British penny was originally minted in silver, until the early 1800’s, when copper began to be used. To not have two pennies to “rub together,” then, could mean to have very little of actual value, since not much precious metal would be contained in a penny, even of silver. And, if the expression references copper or bronze pennies, which contain no precious metal, the allusion would be even more effective, as if to say that one does not even have copper or bronze, let alone gold or silver.