Submitting a manuscript dependent on another one also submitted
Is there a compelling reason to submit both simultaneously? In case (I hope this doesn't happen!) there is an oversight in A, and the reviewer asks for modifications to the method, the results in B may no longer be valid. So it would be better to wait for acceptance of A. 

If you must submit both soon because of deadlines etc, you can cite A as "journal xyz- submitted" (i.e. 'submitted' in place of volume number etc). This does happen now and then, but in my field (materials science), most journals discourage it.