I've taken both supplements but never together. I've read a lot of contradictory stuff regarding the safety or even the effectiveness of using both supplements at the same time. Some searching around yielded the following:
"ZMA contains vitamin B-6, which does not play will with 5-HTP. The B-6 will cause the 5-HTP to convert to serotonin while it's still in the blood, as opposed to in the brain. Serotonin in our blood is not benign - it is shown to cause tumors which in turn can cause cardiac fibrosis."
I don't know if that's true or not, because I have no medical background, but I've seen a load of other comments similar to that. Also, I found the following comment:
"ZMA often has B6 in it (as do a number of other supplements) and B6 is involved in the conversion of 5-HTP to serotonin (5-HT) in the blood. This can be problematic because it's 5-HTP and not serotonin itself that can cross the blood-brain barrier, essentially negatively impacting the efficacy of the 5-HTP component of Z-12."
This would seem to indicate that taking both products together would negate the effectiveness. The last comment was from a pretty respectable source, but once again I have no way of verifying this.