In a conforming C++11 compiler this is safe [intro.memory] (§1.7):
A memory location is either an object of scalar type or a maximal
sequence of adjacent bit-fields all having non-zero width. […] Two
threads of execution (1.10) can update and access separate memory
locations without interfering with each other.
C11 gives identical guarantees (they even use the same wording) in §3.14.
In a C++03 compiler this is not guaranteed to work by the standard, but it might still work if the compiler provides similar guarantees as an extension.