Probably best to illustrate this problem with a sample route.
Say one plans a trip Le Touquet - (overhead) Lydd - Shoreham with alternate Lydd.
Then you want to tweak the leg (overhead) Lydd - Shoreham by 'pulling it' over Seaford VOR.
You can't, because the leg Lydd - Shoreham is overlapped by the alternate leg.
Can anyone suggest a workaround to this problem?