I want to power my Arduino along with 10 meters of LEDs. I have two wall adapters, can I just connect them in parallel to double the output amps?
I would think yes, and it seems to follow my knowledge of circuit theory. But I'm worried about some weird behavior I haven't thought of yet.
Connect one adapter to 5M, connect the other adapter to 5M, use one to power the arduino, connect all GNDs together. Do not connect the two 5V outputs together.
No, connect them in parallel and risk blowing them up, or more likely one does nothing
and the other overloads and shuts down, then the first one overloads and shuts down.
Unless power supplies are designed for master-slave operation you never directly parallel them.
Perhaps you can power some strips with one and some strips with the other? That is
a common approach.
Sometimes it is appropriate to use diodes to connect multiple power sources to a load,
but usually one has precedence - current-sharing is not so simple.
Fundamentally a voltage source (which is what most power supplies are) has zero output
resistance (in theory), and connecting two zero ohm sources together allows very large
currents to flow between them.
In practice one will have a slightly higher voltage than the other and take all the load.
Also sometimes the voltage regulators in the two supplies will interact (each is a high
gain amplifier) and exhibit oscillation - this can over-voltage/destroy everything they
are connected to. You have to analyze the AC behaviour of the two supplies in detail
to be able to predict if this can happen. Low-drop-out and switch-mode regulators are
complex to analyze.