How is a mass, suspended vertically by two springs in parallel, kept stable? Consider a mass suspended vertically from above by two springs in parallel with different spring constants. Wouldn't the tension be different in each spring? How is this system kept in equilibrium?
 A: I've simulated the case 

The two springs had the same initial length, and the block in the picture is in equilibrium. See how it is deviated towards the spring with bigger stiffness to decrease the resultant moment, and stop the rotation of the block.
A: The thing that the springs must have in common is their length $x$. This comes from the mass which is attached to the springs, having different lengths does not make sense in this setup.
From this you can compute the forces. Say the spring constants are $k_1$ and $k_2$. Then the net force exerted is $k_1 x + k_2 x$. In equilibrium, this matches the gravitational force $mg$. Then you have $mg = (k_1 + k_2) x$ which seems to have exactly one solution at
$$ x = \frac{mg}{k_1 + k_2} \,.$$
I do not think that anything is unstable here. Sure, the force that each spring exerts is different. They are $F_i = k_i x$. For different spring constants $k_i$, each spring contributes a different force. The stiffer spring (higher $k_i$) will also exert more force.
