Other people gave explanations on why your model is flawed and hence produces weird results.
I'll give you a mathematical explanation, instead.
TL;DR:
Simply put, you applied superimposition in a subtly wrong way!
Superimposition theorem cannot be applied in your case.
In the following I'll explain what's the catch.
The problem is that many textbooks don't actually teach you the exact, mathematically rigorous, formulation of the theorem (which is a mathematical theorem of circuit theory).
They simply state that superimposition can be applied to any linear circuit, possibly hand-waving away any subtle corner case.
I couldn't find a ready reference online (hep!), so I had to resort to my
trusty copy of the seminal book of Desoer and Kuh "Basic Circuit Theory" (1969). Alas it's a 1991 Italian reprint, so I can't quote the exact theorem in English as the authors wrote it.
Suffice it to say that the theorem has a very important hypothesis which most textbooks neglect: the circuit must have a single zero-state solution, whatever the waveform of ALL the independent sources might be.
Since your circuit has no state (being purely resistive, without any energy-storage elements), it's behavior can be determined using just algebraic equations (that is, no differential equations). Therefore that requirement about the zero-state solution just boils down to the circuit having just a single solution for every possible waveform of the two (equal) generators.
Maybe you might be surprised that a linear circuit might have multiple solutions, but that often happens with "pathological" circuits like yours (usually they are extremely idealized models of real circuits).
Anyway, your circuit fails to comply with that requirement.
In fact, let's call the quantities in the circuit like this:
- V: voltage across the three elements (polarity upward);
- Is1: current through 1st generator (direction upward);
- Is2: current through 2nd generator (direction upward);
- Ir: current through resistor (direction downward).
KVL is trivial here, so it gives us no useful equation. KCL gives us the equation \$ I_r = I_{s1} + I_{s2} \$. Together with Ohm's law,
\$ V = R \cdot I_r \$, those are all the equations of the circuit.
That set of equations has no single solution, since any pair of
\$ I_{s1}, I_{s2} \$ values whose sum equals
\$ I_r = \frac V R \$ will satisfy the system!
You might be tempted to say that, for symmetry Is1 must be equal to Is2, but that is just a physical consideration which has nothing to do with math (perfectly valid for practical circuits, but useless when proving math theorems).
Is1 and Is2 are completely independent from one another, as far as circuit theory goes. Otherwise either one or the other would be a dependent source (and those cannot not be switched off during superimposition application)!
Hence you cannot apply the superimposition theorem to this circuit, because it has no single solution!
