You can deduct the split you pay to your Broker only if the 1099-MISC you receive at the end of the year includes the full amount of the commission (yours plus the Broker's). This would be highly unusual.
In most cases, the 1099-MISC will reflect only the commission that you actually received. In that case, you can't deduct the Broker's portion. (But you won't be taxed on it, either.)
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
