Resource(staff and skill) constraints would prevent this.
it also becomes dangerous when you have more than one person modifying code at a time. It can create bigger issues if it's not kept in check.
Though ones that require content people can obviously be worked on while the coders chip away at something else.
We do however have to remember that these time frames are to fit in with whatever else the cows already do during their workdays.
Oh, maybe I wasn't so clear, I didn't think that they would be worked on simultaneously but consecutively, one from long group then one from short...

