Don't call it a merger unless it really is one. A merger implies two companies of equal value. An acquisition implies a power imbalance. Employees may not like bad news, but they can deal with it. Employees may resent chaos in upper management, but they can understand it.
Deliberate duplicity is unlikely to be forgiven or forgotten.
Announce the company severance benefits as quickly as possible. If the severance benefits are also ambiguous, then many good people are likely to prematurely jump ship as a way of achieving some measure of certainty and control over their professional lives. Establish a Rumor Control Center in each division. This Center could actually be a senior level person who has the authority and credibility to confirm or deny rumors before they begin to spread. Don't jump ship until you are pushed, if the severance plan is decent.