A buddy of mine I worked for a beer distributor in KC that merged with another about 5-6 years ago. Sort of the same situation, it was worded as a merger but the bottom line is that one company paid the other to make it happen. Both distributors were union, same local but all of the drivers at his shop were put underneath the drivers of the other once the deal closed. Once the routes were all combined and reworked some were let go for lack of work, most of the others quit shortly after because of the lost seniority and the way their pay was changed from piece pay to % of sales like the other company paid before the deal. Granted, the business agent with that local was formerly a driver with the other company so he pretty much green lighted whatever they wanted to do despite the protests, but that's how it went down there. US has a drop yard here in town (warehouse is 70 or so miles away), for what it's worth about a week ago we got a memo asking us not to talk to any of their employees about seniority or job security.
Given the union/nonunion situation with both companies and the fact that there's no master agreement I bet how they handle this varies significantly from one location to another. Given the volume of groceries involved I can't believe they're going to be handle it in either company's existing buildings alone. Some will close, but I bet a lot won't...maybe rework or tighten up current warehouses coverage areas?
That's assuming the FTC even allows the deal to close. I read somewhere online that there's a clause that allows for a one time lump sum payment to US if the deal doesn't or isn't allowed to close as planned. Assuming that's the case then the lawyers that drafted it feel that it's at least a possibility.