Why do I need witness in WSFC with two nodes?
Straight from the docs, with a two-node cluster without a witness:


  One node's vote is zeroed, so the majority vote is determined out of a
  total of 1 vote. If the non-voting node goes down unexpectedly, the
  survivor has 1/1 and the cluster survives. If the voting node goes
  down unexpectedly, the survivor has 0/1 and the cluster goes down. If
  the voting node is gracefully powered down, the vote is transferred to
  the other node, and the cluster survives. This is why it's critical to
  configure a witness.


Understanding cluster and pool quorum

And you can simply use Cloud Witness, so you don't need a shared disk, third node, or file share.