The boundaries of a bargaining unit are set by the location of work and the job classification. These are obviously fuzzy categories and sometimes the National Labor Relations Board has to step in and decide which subset of workers gets to be in a bargaining unit. However, there’s just nothing in the law that lets workers say “OK, everyone with ten years or more seniority will be represented by Union X, and everyone with less seniority will be represented by Union Y, and we will negotiate separately for contracts.”