I ask this question, because at the end of this long day I'm just too dazed to derive the proofs myself (even though I know that I should feel ashamed for this).
So, the question:
Given two simple gravity pendulums, attached to the same hinge point. Both masses and lengths of the strings (or rods) are equal. The only difference between the two is that the motion of one pendulum takes place in a plane perpendicular to the motion of the other.
So, supposing that initial conditions are chosen such that these pendulums never collide, can the position of the center of mass of both pendulums be described by the motion of a single pendulum with twice the mass, the motion of which is not restricted to any single plane?
Intuitively, I'd say: perhaps only for small angles. For example, I can easily imagine one pendulum going one-way (spinning a full 2$\pi$ per period), while the other is at near-standstill. Obviously, the motion of the COM can not be described by a single pendulum without significant, non-physical changes to that model (the periods of both are unequal in that case, generally meaning the center of mass moves along non-periodic Lissajous patterns in $x$,$y$,$z$.).
So, what I'm really asking is: under what conditions is this possible, if at all? What are the constraints and/or adjustments you'd need to make to this "replacement pendulum" model for it to work?
And how does this all translate to physical pendulums?