I'm going to be contrary and say that the horizontal piece you're thinking of removing currently contributes nothing to the structure. (And I don't think those trusses were engineered, just built.) A rafter transmits the weight of the roof to the wall. Since the weight of the roof is centered over the center of the roof and not in line with the walls, the difference creates a turning motion in the rafter that causes the rafter to push outward against the wall. Tension in the collar tie balances the outward push of the rafters.
The upper horizontal piece is not needed as a collar tie. The lower rafter, the lower horizontal piece and the vertical piece form a triangle, any horizontal force imparted by the upper rafter is transmitted to that triangle and ultimately to the lower collar tie. So long as the joint where those three pieces meet is strong enough the lower collar tie does the whole job.
What are the magnitudes of the forces? Let's go big and assume a roof load of 60 lbs per square foot. (This is not the everyday load, it's the "Storm of the Century" when there's two and a half feet of wet snow on your roof. Everyday load is probably about 10 lbs/sf). With a span of 22.75" and a spacing of 32 inches each lower rafter supports 728 square inches or 5.05 square feet. We'll call it 300 lbs. Each upper rafter has a span of 43" and supports 1376 square inches or 9.5 square feet, or 630 lbs. At the angle of the lower rafter, a vertical force of 300 lbs at the midpoint requires a horizontal force of 72 lbs and the end to counteract the turning motion. The upper rafter requires a horizontal force of 270 lbs to counteract the vertical force of 630 lbs. At the place where the lower rafter, upper rafter and vertical piece meet the combined loads are 630 lbs vertical and 342 lbs horizontal.
If the piece you are considering removing were acting as a collar tie, at most it would be contributing 270 lbs of tension -- the entire horizontal force of the upper rafter. Removing it shifts that horizontal force to the lower rafter. In either case the vertical force of the upper rafter is borne entirely by the vertical pieces, removing the piece changes is nothing for it. So the question is whether the joint of the two rafters can handle an additional 270 lbs of horizontal force. If the joint is wood on wood in the direction of the force, 270 lbs is nothing, all you have to worry about is the joint slipping. If the rafters are nailed to sheathing they aren't going to slip. If you're relying on nails to prevent horizontal movement I find a good rule of thumb is a 16 penny nail can take 200 lbs of perpendicular force.
If your drawing is accurate the the bottom ends of both rafters are horizontal I would worry about their ability to transmit horizontal loads. Gussets there would help.
The joint at the ridgeline does not have to be modified. The downward force of the weight of the roof tends to push pairs of rafters together, the rafters just have to be fastened well enough to keep from slipping. The placement of collar ties does not change the forces on that joint in any way (as long as the collar ties are sufficient to keep the building from falling down).
