Try using the search engine next time. But to sum it up:
5-HTP is converted to serotonin in the body. Not limited to the brain, but the gut, liver, blood stream etc...
Free serotonin in the blood is bad for you, activated peripheral 5-HT2B (serotonin) receptors is what made diet pills cardiotoxic.
B6 is a co-factor in the metabolism of 5-HTP to serotonin.
Taking more B6 increases the rate of this reaction. It will probably increase the reaction most in the gut, 2ndly in the liver, 3rdly in the blood and last in the brain (i.e. the order in which the vitamin is absorbed and distributed).
Therefore, taking B6 with 5-HTP increases the amount of 5-HTP converted to serotonin outside the brain (i.e. gut, liver and blood). This is bad. It could just mean you waste the 5-HTP, and are more likely to get gastrointestinal side effects... however, it could also be bad for your heart.
No one knows the answer to these maybes.
Conclusion: There is no advantage to taking 5-HTP with B6