The rule says spell effects don't combine. For two Booming Blades cast by a level 1-4 you'd get:
Booming aura + booming aura = booming aura. Both effects still exist but one of them effectively is ignored. It's like 1 + 1 = 1. The effect is still there. It's just not doing anything.
Once the trigger happens (moving) you get:
1d8 + 1d8 = 1d8. Both effects still happen but one is ignored because the effects can't combine.
Then you get:
Spell ends + spell ends = both spells end. This happens because "spell ends" is internal to the spell. The effect affects the spell not the target so the prohibition against combining doesn't apply.
I'd be curious to see what game designer logic on this is.
Booming aura + booming aura = booming aura. Both effects still exist but one of them effectively is ignored. It's like 1 + 1 = 1. The effect is still there. It's just not doing anything.
Once the trigger happens (moving) you get:
1d8 + 1d8 = 1d8. Both effects still happen but one is ignored because the effects can't combine.
Then you get:
Spell ends + spell ends = both spells end. This happens because "spell ends" is internal to the spell. The effect affects the spell not the target so the prohibition against combining doesn't apply.
I'd be curious to see what game designer logic on this is.