Hi Rob,
Making a 2 part resin mold is tougher than it seems,
problem is, resin will shrink somewhat while curing & this leads to problems with the 2 parts not lining up with one another, or even running into one another.
Theres a way to do it but the way i do it requires an RTV master/mother mold (a mold of my mold) then I mix enough resin to fill both halves of my mold & pour them both at the same time, this allows the halves to cure at the same rate & since the resin is mixed equally it usually cures evenly allowing the 2 halves to match.
RTV has no noticible shrinkage, nor does plaster or putty those might be other options for ya.
If you had a rigid copy of your bait (instead of soft plastic) you could bury half of it in clay & then cast resin over it.
If you still wanna try it I recommend you do like you said:
mold the whole bait, but dont overdue the catalyst & cure it slowly in a cool place. Heat & over-catalyzing will encourage more shrinkage.
Sand away half,
Drill your keyholes,
Apply a wax based mold release to the sanded face of the mold.
Replace your bait in the half cavity & pour your second half.
Good luck bud, its all a learning experience.