Each attack is a separate source of damage
When you have the Extra Attack feature, each attack that results from it is a complete attack that is resolved distinctly from the other. The PHB outlines how each attack resolves:
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In
addition, spells, special abilities, and other effects can apply
penalties or bonuses to your attack roll.
Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise.
Some attacks cause special effects in addition to or instead of
Thus, after each attack you will roll and determine damage. If any one of those attacks meets the criteria for Massive Damage, they target will be killed instantly.
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
Notice that you consult the Massive Damage rule whenever you take damage. And you take damage at the end of each attack. So, since each attack is resolved separately, the damage from multiple attacks is not added together for this purpose.
Scenario 1 - not dead
Garry is not dead, since neither individual attack did enough damage to count as Massive Damage.
Garry will have one or two failed death saving throws depending on if the attacker was within 5 feet or not (due to the critical hit rule).
Scenario 2 - not dead
If multiple creatures Ready attacks, each attack is still resolved separately. So this does not change anything with regards to Massive Damage. Since each attack did less than 18 damage, none of them kill Garry outright.
However, Garry does now have two failed death saving throws because they have been hit by two attacks while at 0 HP.