[Lizard]

The only way I can think to do it at the moment would be rather manual.
Create 3 tasks:
'Dig small hole' -- assigned to person A
'Dig big hole - phase 1' -- assigned to person B (should have approximately the same end date as 'Dig small hole')
'Dig big hole - phase 2' -- assigned to person A & B
Make sure there's a dependency from each of the first two tasks to the third one.

Now for the fussy part:
When 'Dig small hole' is done, edit the end date of 'phase 1' to that same date and set its completion to 100%. ['phase 2' should slide over to start immediately]
Update 'phase 2' so that its effort reflects the remaining amount of work for digging the big hole.
From here, you can treat this as a normal task.