This is already implemented in the Game and I use it all the time - switch the Crossbar mode to Toggle or Mixed and you'll get what you're looking for. @frdtma - it's actually less cumbersome as you don't need to hold multiply buttons at the same time. It's very good for crafting. Also you can switch targets by using L1 / R1 while the cross-bar is "toggled in" which is much easier than pressing L2/R2 + L1 or R1.

It's in the Character configuration screen.