C++ :: Two Identical Storage Containers - Typecasting

I have a situation where I have two identical storage containers:

Code:
////////////// multiplatform version
union _SOVector3 {
struct { float x, y, z; };
struct { float r, g, b; };
float v[3];

[Code] .....

SOVector3 is part of a namespace with specialized functions that are generic and intended for multiplatform usage.

GLKVector3 is dedicated to the Mac and has its own set of functions.

But what I want to do is freely interchange the storage between these two namespaces. Such as like this:

Code:
start = clock();
SOVector4 myVec4 = SOVector4Make(1.0f, 3.0, 6.0f, 1.0);
SOMatrix4 myMat4 = SOMatrix4Identity;
for (uint i=0; i<100000; ++i ) {

[Code] ....

But I am getting errors when I typecast this.