Car adapters (12V -> 5V) in parallel
It depends. If the adapters are powering separate circuits, it should be fine, provided you're not exceeding the current rating of the wires. 

However, connecting the outputs in parallel would not be a good idea, as any voltage difference in their outputs will place more of a load on one, possibly exceeding its rated current. You almost certainly will not be able to safely produce the sum of the two adapters' currents, as the weaker one will likely drop its output voltage, leaving the other adapter with most of the load. In this case, you're much better off buying a single large supply that can handle the current you need.