Explanation:
Plasmids are circular pieces of DNA that exist outside of the bacterial chromosome, and they can carry genes that provide specific advantages to the bacteria in certain conditions. One such advantage is antibiotic resistance, which allows the bacteria to survive in the presence of antibiotics.

When scientists want to introduce a new gene into bacteria, they often use plasmids as vectors to deliver the gene. This is called transformation, and it can be a useful tool for genetic engineering.

Having two antibiotic resistant genes in the same plasmid can be helpful in identifying transformants, which are bacteria that have successfully taken up the plasmid and incorporated the new gene into their own DNA.

Here are some reasons why having two antibiotic resistant genes in the same plasmid can be useful:

- Selection: If the plasmid carries two different antibiotic resistant genes, then scientists can use two different antibiotics to select for bacteria that have taken up the plasmid. This increases the chances of finding transformants, because the bacteria that are resistant to both antibiotics are more likely to have the plasmid with the new gene.

- Identification: If the plasmid carries two different antibiotic resistant genes, then scientists can use a combination of both antibiotics to identify the transformants. This is because only bacteria that have taken up the plasmid will be resistant to both antibiotics, and this makes it easier to distinguish them from other bacteria in the culture.

- Functionality: If the plasmid carries two different antibiotic resistant genes, then it is less likely that one of the antibiotics will be ineffective. This is because some bacteria can acquire resistance to a single antibiotic through a mutation or other mechanism, but it is less likely that they will acquire resistance to two different antibiotics at the same time.

Overall, having two antibiotic resistant genes in the same plasmid can be a useful tool for genetic engineering, because it helps to increase the efficiency and accuracy of the transformation process.

In general genes that confer resistance to various antibiotics are used as selective marker in cloning vectors.