That just means you'll be taking a DOT-regulated test. The testing process is regulated by the US Department of Transportation. With DOT-regulated testing, there are regulations and protocols that must be adhered to by all individuals in participation. The regulations themselves are found here: Procedures for Transportation Workplace Drug and Alcohol Testing Programs | Department of Transportation

They split my void into two vials..
This is mandatory on all DOT specimen collections. DOT requires only 45ml of urine sample. The sample is then split 30/15 (into bottle A and bottle B) and both are sent off to a lab. Bottle A gets analyzed while bottle B is put into storage. If bottle A is confirmed positive for a particular drug(s), and the donor decides to mount a challenge to the results, then bottle B gets tested.

Edit: Since youve just pointed out that your test was DOT, here is the regulation outlining what an employer is to do when s/he receives a "diluted" result:
http://www.dot.gov/odapc/part40/40_197