Alternating flashing LEDs with a just a single mechanical switch can be done. This circuit is probably not practical, but building it helps understand electricity better.
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The circuit as shown in the diagram will work well with a 5V supply for Vcc. Higher voltages will likely need more than 220Ω from Vcc to the switch if the current/power limits of any component will be exceeded.
2 LEDs of the same color will need twice the voltage to conduct as a single LED with little to no resistance added to it. Switching the single LED on drops the voltage across the 2 LEDs enough to turn them off.
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