Heh see If I can use words to paint a picture
Butt together the two 2x6. Place a 3ft 2x6 evenly over the butt joint. Use carriage bolts to fasten the 3ft board to other two boards. Two carriage bolts at each end of the 3ft board. One more carriage bolt in the middle of 2x6 @ 3" about 10" from the butt joint.
Picture a long length of 2x6 with a small length of 2x6 attached at its middle.
Use two of these with a 2-3ft width of 3/4" exterior grade plywood.(Use real ply not other stuff

) A width can place a foot down and the bike is walked up the ramp.
A safety chain connecting the ramp to the truck so it doesn't shift off the tailgate.
Added strength by using a tension cable under the 2x6. Put a spacer in middle of the 2x6 board to get an angle for the cable. A block made of multiple pieces of 4x 2x6 will do it. Use a metal plate near the end of each 2x6. Mound the plate using multiple wood screws. Drill one 3/8" hole through the 2x6 and plate. Use a carriage bolt with two large washers and nut. Make up a 1/4" cable the length of board plus one turnbuckle. Tighten the cable loop around the 3/8" bolt between two large washers. Tighten the carriage bolt having the cable leading towards the center of the board. Do the same with other side and connect the turnbuckle. Tighten the cable to get a twang sound when plucked.
I know there is assembly required everytime its used. If the boards are mounted to the plywood. Having two shorter length of ply is not unreasonable. The center 2x6 lapping board and then cable connecting is all needed to do.