Most likely you'll want a code solution. I can't think of any way to accomplish this with the stock Quake entities. Unless you can stack one entity inside of another. I think Quark allowed this but I don't know how it resolved it in the compiler or game.
Now, it has to be said that two triggers *CAN* share the same bsp model, but they will still appear as two separate instances of the same brush. For example, in code you can say newmis.model = plat.model (which ends up being "*4" or something similar) and newmis will look like the platform. However, moving the platform doesn't move newmis and moving newmis won't move the platform.
One easy QC solution might be to have your trigger (that destroys the object thrown in) point to the object it should follow (using .target or some other field). In code you make sure it links to the other object. Then it sets the destroyer trigger to MOVETYPE_FOLLOW so it automatically follows its target around. If you can't assume MOVETYPE_FOLLOW is supported by the player's engine, then you can always do it the manual way with a think function setting its origin every frame or 0.1 seconds. Because you're working with BSP models there's some weirdness there with setorigin, but since they're both BSP models (presumably) it should work without problems.
I think I understand what you're trying to do, but I might not. I hope my suggestion is helpful to you.

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