Good photo, you may "luck-out" because you've smeared heat-sink grease, and because you've not clamped regulators to heatsink with a screw and insulating shoulder washer. Bad luck may result in blown 5v regulator, or +12v finding its way to your +5v output terminal - the heatsink electrically connects both outputs together if the insulating heatsink grease fails.
I've "lucked-out" doing as you've done (but I've clamped down those regulators to the heatsink carefully).
About heat-sinking...you have a good big heatsink, but those regulators are a bit too close to each other - one can overheat the other. If you can keep a finger on each regulator while they're working hard delivering power to a load, then heatsink is likely adequate. Your +5v regulator has a larger voltage drop from un-regulated input to regulated output, so it may get hotter than the +12v regulator for similar output current. I'm assuming that both regulators have their inputs fed from the same DC voltage source.