In the case of state laws, there's usually some communication and cooperation between them in established code. For instance, 2340B has "Nothing in this chapter shall be construed as precluding the application of State or local laws on the same subject, nor shall anything in this chapter be construed as creating any substantive or procedural right enforceable by law by any party in any civil proceeding." State laws couldn't say the same thing without being laughed at, though.
Whether or not a federal law begins to be enforced in contradiction to a state law comes down to whether or not action is being taken (the crimes are being investigated and
something is being done to stop illegal activity). I believe this is where jurisdiction comes in, in that someone has to take it, and if state law doesn't allow for taking it (it's legal), federal enforcement gets it. If you have someone violating a federal law and state law even explicitly legalizes what they're doing, they've still committed a federal crime and are subject to the punishments federal law has for them.
Actually, considering this, smokers should probably be a little wary of legalizing it on the state level, because that would easily enable you to be charged on the federal level. Federal prisons are not fun-times, unless the alternative is Texas or something, I dunno.
Also:
Eagleon, if this is as you say it is - why aren't the feds knocking on the doors of Texas executioners to drag them to trial? (I believe it's because local law is the only law that's applicable, and it says murdering people under those conditions is a-ok)
It's a bit different when it's an official action (I think? O.o). And federal law actually still permits the death penalty for certain things, including murder. edit - from 1111, "(a) Murder is the unlawful killing of a human being with malice aforethought." So somewhere in federal law is codified when it is lawful, and presumably states get some of that lawful killing action along the way. I'm nowhere near lawyer enough to say for sure, but I believe it's something like that.