Well, even if its D&D 5e rule, is it really a good game mechanic with spells overlapping and preventing you from using them? I find both very limiting.
The issue is that changes to balance have to be done carefully, otherwise seemingly small changes can cause major issues with balance. Just look at the cantrips. I've done 13 damage from a single Firebolt, which is supposed to do 1 to 10 at most. Ray of Frost can instantly knock a target prone. Acid reduces AC by 2 - which is not in the game. Shove and Disengage-Jump as free actions make it too easy for squishy characters to avoid AoO and makes positioning unimportant.
Spellcasting is especially vulnerable to changes, as it is very powerful. Two concentration spells at once would be a major, balance-breaking buff to casters.
