When they picked 400 apples each in the first round, they picked a total of 2*400 = <<2*400=800>>800 apples.
They picked 3/4*400 = <<3/4*400=300>>300 apples each in the second round.
The total number of apples they picked in the second round is 2*300 = <<2*300=600>>600
The total number of apples they'd picked so far is 800+600 = <<800+600=1400>>1400
When they checked the pickup truck that was carrying the apples, they found out they still needed to pick 600 apples each to reach the target number, a total of 2*600= <<2*600=1200>>1200
The target number of apples they had to pick is 1200+1400 = <<1200+1400=2600>>2600
#### 2600