Extending 2x4 joist to square up shed foundation
You are correct to worry that the sistered joists connection to the post is weak. What's supporting that corner is the shear strength of the 4 framing nails above the post, the friction of the 2x4s being compressed by the bolts, and the levered shear strength of the bolts. You could improve it by:


put a bolt through the 2x4s above the beam.
add 2x4 blocking below the 2x4s and nailed to the post
add framing angles with structural screws, fastened to the post and the outer 2x4s


Something else I don't see is attachments to keep the 2x4s atop the beams. You most likely want some joist hangers or hurricane ties to maintain that connection.