Semi....
You don't have to heat the halves you insert before the final injection, but you may find it necessary to keep the heat of the injected plastic upward of 320-340 degrees. The greater the area to weld, the higher the heat is my rule and when the injector rod stops when the mold is filled you'll want to keep some light but constant pressure on it for up to a ten count.
Also, I recommend making the pieces as you go, meaning if you want to make twenty frogs with bellies and backs different colors you make enough pieces to get it done at one session. Stockpiling pieces, say frog belly parts, in a bag for use down the road a couple months will hand you problems. As plastic sits it will exude an oil and if you do not get all of that oil removed from the surfaces you need to weld they will delaminate real easy. Small parts, like tails, you can get by with by leaving the tails longer than what you need when you shoot baits using them and simply cutting them to length as you re-insert them in the mold. This allows for a fresh surface for the second plastic to weld to.
Another tip in this process is to slow the injection of the second plastic down so you don't lift the inserted piece with the incoming plastic and under-shoot it.
This isn't a hard process but these little hints will make life easier for you as you move ahead with it.
