Two positives to one accessory?

This is probably a very basic question but one that I do not know the answer to. What would happen if you run two positive wires to a device? Say, for example, you had an LED that you wanted to be able to power via two separate switches.   For simplicity's sake, say you ran positive wire 1 from battery to switch 1 to LED, and positive wire 2 from battery to switch 2 to the same positive terminal on the LED. The LED would light up if either switch were turned on obviously, but what would happen if you turned on BOTH switches, either at the same time or separately?   My untrained mind tells me that it shouldn't matter, but I suspect this would cause some sort of problem. What would happen if you did this, and why?

Nothing, in the real world, either relays, diodes etc. would separate the sources..

The problem is what else is connected to the switches? They would also be powered with either switch. Closing both switches has no problem other than interconnecting the two switch sources - eg, IGN to ACC or +12V, or fuse #1 to fuse #2. The LED solution is a diode from each switch output with lines towards the LED+. 1N4004s or 1N4007s are the common diodes used but smaller diodes like the 100mA 1N914/1N414 would handle 20mA LEDs etc (ie up to 100mA).

It wouldn't be a problem with simply switches, once you use TTL logic to drive an output, that's when the diodes become necessary.

It would be clearer with a diagram, but I interpreted it as simple OR logic - ie, the LED is on whenever switch 1 or 2 is on, hence the diodes. But I'm leaving the loads out of the answer. If it or they can also be DIODEd, fine, but otherwise it gets complicated. Maybe it's a lighting circuit with just 2 switches driving a slightly dimmer LED and nothing else...

Here's my attempt at a basic diagram. I infer from the responses that this diagram would pose no problems if wired up exactly like this, as the power source is just the battery to both switches, and by closing the two switches there is no issue created when the currents going through each of the switches "rejoin" when it hits the light. If, however, I introduce a different power source (say tapping into an ACC wire like the cigarette lighter or tapping into the brake light wire), it would still be OK using each switch separately, but a diode would be needed to keep the power streams separate if both switches could be turned on at the same time. If no diode was used, then is the only "bad' thing that happens that the battery power would go into the brake light postive wire and turn on the brake light? (Note, I am not proposing to wire something this way, just trying to understand a basic element of 12VDC power.)

As shown in your diagram there is no problem - they are simply 2 switches in parallel. If you power switch 2 from another source whether another battery or ACC or brake etc then you interconnect both sources/supplies when BOTH switches are closed. If you can do that without unacceptable consequences then fine. Otherwise you need diode isolation. (EG to stop the brake light coming on when switch 1 is closed, or powering your IGN or ACC when both switches are closed; or interconnecting 2 batteries (which may be ok) etc. Diode isolation is fine provided the load(s) are fine with the diode voltage drop (typically 0.3V to 0.7V depending on the diode for low currents) AND the diode can handle the load - ie, a single 23W stop lamp requires a 2A diode (hence use a 3A diode) whilst two 23W bulbs require a 5A diode.

Why 2 switches? Are they in 2 separate locations? If you want control from either location get 2 single pole double throw On-On switches and wire as a 3 way switch. This way you can turn on or off from either location.

Thanks for all the replies. i am an idiot - I used to work with AC electricity and have only a basic understanding of DC, so was trying to further understand this Agree totally that a 3 way switch would be best if I actually wanted to run something like that. One last question if I can ask - when it comes to running diodes, all of the diodes I see for sale are ones that look like you would use in a breadboard - long metal leads on either side. How do you install these in a car? Do you solder them in-line then simply cover it up with heat shrink tubing?

Yes, solder & heatshrink is best. Not that the diodes are in line - they'll be parallel ie joined at one end with their other ends to whatever circuit (switch output etc). AC & DC are fundamentally the same wrt circuit connections - ie electricity (current) flows in a loop, and usually there's a hotside and a coldside - eg active, +V, +12V etc & GND, earth, 0V (aka batt-). A 2-way DC circut using 2 SPDT switches is wired the same as the AC equivalent. (Tho the dc version doesn't have the same "dangerous" possibility that the 110V/230V AC version has...) The main differences include that you cannot isolate AC signals/sources using diodes, high current dc is difficult to break, and high voltage DC is a lot more dangerous. But you can touch +12V & +24V etc systems without fear of electrocution... in general. (Noting that in one particular year, 3 "accidental deaths" downunder here were attributed to 9V batteries.)