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.