I believe everything BelknapPoint said is correct (and comprehensive) but I think even if you can rollover 529s into one account, it is a very bad idea to do so. A rollover may have gift tax consequences. Moreover, rollovers have a 12-month clock, and if you run afoul of this, a whole rollover could become an unqualified distribution, with tax and penalty consequences. Just don’t mess with rollovers unless there’s a really good reason and you fully understand the rules to avoid pitfalls. Just leave the accounts as they are, and spend from them separately.
By the way, if you ever need to change beneficiary, that’s quick and easy to do, and doesn’t have the abovementioned pitfalls. (Also, get ideas from internet strangers, but check the rules yourself.)