There is no particular qualification to be allowed to provide a service of installing dual controls. However if you buy he man dual controls then they have a list of approved fitters. Use one of these dual control fitters and you can be sure that your he man dual controls will be correctly fitted.
Of course He Man are not the only manufacturer of dual controls. There are several alternative dual control manufacturers. However heman are the most well known and ar effectively the industry leader.
If you are looking for a he man dual control fitter the go to the heman website or give them a call and they will be happy to help you.
If the cost of he man dual controls looks a little pricy (around£500 fitted) depending on your vehicle, then you can always look at alternative suppliers such as Pajedo.