Regardless of the number of queue readers, items in the queue will be picked up in a FIFO manner. From the Service Broker point of view, it will always be logically correct.
However, you'll need to test the actual work inside the activation procedures to ensure your process still remains in sequence.
A somewhat facetious example:
create procedure pInsert
waitfor delay '0:00:20';
insert into dbo.table1 values ('blah', 'blah', 'blah');
create procedure pDelete
delete from dbo.table1 where column1 = 'blah';
It *is* possible that your delete could outrun your insert and produce undesirable results...