Predators have favourite spots to eat their prey, so the first two in the same spot would suggest a Sparrowhawk (eat on the ground if prey too big to carry off... will often fly off with the remains after eating half of it) or Cat. But you've not said that there were feathers everywhere, or other signs of a struggle, so any predator is unlikely. Crows hate Magpies, and will kill any they can, but again, you've not reported any signs of a struggle, and Crows mostly club them to death with their bills, so it can be messy.
The two together in the same spot is odd, so only leaves a few options (one already mentioned):
- Someone is shooting them with an air rifle or catapult.
- They are being poisoned, perhaps from poisoned bait elsewhere.
- Someone is trapping and killing them elsewhere and throwing them over your garden.
I've ruled out them being killed in their nest/roost and falling to the ground below, as you've not described and signs of a struggle (birds tatty, bloody, missing feathers etc.)
(Magpies are a much-maligned species and are blamed for practically everything, including much that other Corvids do (Jays probably kill more fledglings than Magpies, but Magpies are nearly always blamed). OTOH, if you had Mice in your garden, Jays would mostly ignore them, Magpies would kill as many as were stupid enough to show themselves in daylight. So much-maligned and often misunderstood. They are just a bird, like a Jay, but not as "pretty".)