hmm. u may try my friend style. but a bit cb.
He accept company B. still got a. then when found out a better, tell b not joining. As by right the letter of appointment will indicate that if first day no come for job mean dont want. but u can call earlier also. my friend do this and when company a cfm him he say he can start work ealier as when he accept company b he alraedy resign from current company.