Writing an iterator that makes several containers look as one

Consider the following simplified example and desired output:

class A
{
    class combined_iterator
    {
        ????
    }
    typedef ??? t_combined_it;

    t_combined_it begin();
    t_combined_it end();

    std::vector<int> m_Vec1, m_Vect2;
}

A a;
a.m_Vec1.push_back(1);
a.m_Vec2.push_back(2);
for (A::t_combined_it it = a.begin() ; it != a.end() ; it++) {
     std::cout << *it << " ";
}

Output:

1 2 

I think the question is clear from this: how do I write an iterator that makes it look as if two or more other iterators are really just one sequence. So that, in the example, instead of iteration over both m_Vec1 and m_Vec2, I can use an iterator that iterates over first the elements of m_Vec1 and then m_Vec2, transparently.

I found the following question which I think asks the same: Make a c++ iterator that traverses 2 containers . There were no good answers to this question; the solution presented by the original asker seems convoluted, and it is (relatively) memory-intensive.

I tried a naive approach by keeping a std::vector::iterator as a member of my custom iterator, and comparing it to the .end() iterators of each of the sequences being iterated over; however it seems that it is illegal to compare iterators from different containers (where I would have preferred them just to return 'not equal' - maybe that is a direction to go for finding the solution to this problem? I can't think of how to implement it, though).

Where possible and if relevant, I would like to use boost::iterators as I use them elsewhere and I like the homogeneity it provides to my iterator implementations; but of course if someone has an idea without using them, I can work them in myself, so they're not required in that sense.