One H1B petition cannot be transferred to another employer. You may use an approved H1B petition to file another cap-exempt petition with another employer. Since your Company A petition is still in "Initial Review", you cannot use that for filing a cap-exempt petition with Company B.
Once approved, you can use the I-797 notice of approval, I-129 petition copy and the notice of receipt (any one of these would suffice, I think) can be used to file a cap-exempt petition with Company B.