I was always taught to do a 'short' jab for this first one and then a long jab.

The first jab will push the target back so you need the longer shot.
Other way round and the second jab isn't gonna connect as hard or at all.

Used to have the same problem with one twos... have to get that off really quickly on the heavy bag... the first jab will move the bag so I've got to get in there with the cross quickly or I miss the bag and lock my elbow...