If you are talking about connecting 2 power supplies to the same chain of blades, you will have a bad time. You will effectively be linking the grounds of both power supplies to each other and linking the +12v of both power supplies to each other. If either of these have a voltage difference, bad things will happen and power will flow to the lower potentials unrestricted.
Switchmode PSUs have feedback to regulate the voltage. Connecting two in parallel that have slightly different voltage would just cause one (the one with lower voltage) to just not provide any power (or even shut down due to broken feedback loop or overvoltage). The current cannot flow into the output of the PSU just because there is nowhere for it to go. Still, connecting two power supplies that were not designed for it in parallel is not a good idea.
Two unregulated linear power supplies (basically transformer->bridge->cap) in parallel would just work and share the current a bit. If the power supplies are regulated then it depends on the linear regulator.
Now, connecting two batteries in parallel will cause the higher voltage one to charge the lower voltage one.
