If you need to span a distance greater than your lumber dimensions, scarf joints are the way to go. Another variation on the scarf joint, this stop bladed scarf joint is a half-lap joint with the addition of the stops, also called tongues or blades. The stops prevent twisting and also improve bending and tensile strength. If you need to lengthen either a sill plate or roof plate this joint would be a good choice.
You can use a chain mortiser to notch out the blades, making cutting this joint this fairly straightforward.