If one of my monsters is destroyed and I have two Interplanetarypurplythorny Dragons in my hand, can I summon both of them from that one monster destruction (assuming I have space for both monsters)
Yes you can
The other answer is not correct (although a comment under the answer points to the correct solution).
The reason is that Interplanetarypurplythorny Dragon's effect reads "If a monster", and it does not read "When a monster". Cards that read "if" do not miss timing and can be activated at the first opportunity after the effect occurs.
From the missing timing Wiki Page, emphasis mine:
Some Trap Cards, Quick-Play Spell Cards, and optional effects must be activated in response to a particular event (such as the Trigger Effect of "Peten the Dark Clown"). These optional effects are marked by the usage of the word "when" in their activation timing. Trap Cards, Quick-Play Spell Cards, and optional effects that use other words instead of "when" (such as "if" or "each time") do not need to be activated in response to the event, but simply at the first opportunity after the event occurs. Likewise, mandatory effects do not need to be activated in response to a particular event, always being activated at the first opportunity after the event occurs (regardless of whether they use "when" or not).