Concatenation Physical Operation: Does it guarantee order of execution?

Question

In standard SQL, the result of a union all is not guaranteed to be in any order. So, something like:

select 'A' as c union all select 'B'

Could return two rows in any order (although, in practice on any database I know of, 'A' will come before 'B').

In SQL Server, this turns into an execution plan using a "concatenation" physical operation.

I could easily imagine that the concatenation operation would scan its inputs, returning whatever input has records available. However, I found the following statement on the web (here):

The Query Processor will execute this plan in the order that the operators appear in the plan, the first is the top one and the last is the end one.

Question: Is this true in practice? Is this guaranteed to be true?

I haven't found any reference in Microsoft documentation that the inputs are scanned in order, from the first to the last. On the other hand, whenever I try running it, the results suggest that the inputs are, indeed, processed in order.

Is there a way to have the engine process more than one input at a time? My tests (using much more complicated expressions than constants) are on a parallel-enabled 8-core machine, and most queries do take advantage of the parallelism.