I have a 2x6x18' CRACKED roof rafter that I want to sister. Can it be done in sections? I need to go from ridge to top plate. Impossible to get an 18' piece into my attic.

The pieced in sister rafter will not be any stronger than the longest piece you can get into your attic. Sandwich the cracked rafter with two 2x6 one on each side glued and nailed, the longest you can slide into the attic. You might try to jack the old rafter outward at the crack with a diagonal support attached to your ceiling joists. Attach this above a wall in the room below and not in an area that is just a ceiling, you might bow the ceiling.

Is the rafter part of a truss or just an 18' 2x6 spanning the roof? I assume this is a peak roof not a flat roof?

The rafter with the split is actually 16' and is on a slope and is not part of a truss. I can get a 16 footer into the attic via a bedroom window and into the attic. My question is: Can I screw one 16' board to the split one and put some lag bolts through both?

I can't visualize how you can have a 16' rafter that is on a slope. What is supporting the ceiling beneath it? The ceiling is sloped too?

Structurally, houses are pretty simple. Some wood carries the floor load/ roof load. This needs to be wide enough to withstand the flexing. The forces on these pieces are straight down from the weight put on the floor/ roof & gravity. All other structural wood is either in compression or tension with the forces going straight along the length of the wood.

Walls are compression because they support the weight of the floor/ roof.
Horizontal rafters are in tension because they stop the outside walls from moving outwards. The only vertical load that rafters carry is the ceiling underneath them, and the crap you put in your attic for storage. That is minimal.

Bottom line is that you should be able to sister on pretty well any length of wood onto the rafter. Longer is better, but trying to fit the absolute largest piece is overkill. Lag bolts are more than enough to carry the tension. It would be nice to have one end of the cripple supported by the outside wall, and the other end supported by an interior wall - but remember this is only to stop any sagging in the ceiling underneath so the interior wall does not have to be a load bearing wall.

If the 16' piece is on an angle and does not support the ceiling below, then it must have a rafter below it and it is part of a truss. Trusses refer to a general roof design and not just those prefab roof trusses. (Do they slope to alternate sides of the roof between rafters?) If this is the case, whether it is in tension or compression doesn't matter... just lag bolt your cripples in and you'll be fine.

Sorry for the confusion. The 2x6x16' rafter is one rafter in a truss system that goes from ridge to top plate. It would be a lot easier slapping on shorter pieces but I thought if I screwed on one 16 footer from top plate to ridge beam into the split rafter it would be stronger.
Thanks for your patients and help.

Skip the lag bolts and go with nailing and wood glue. You can do what you describe but the lag bolts will tend to slit the lumber more than nailing and glueing.