To begin with, a 2 - in- 1 shampoo and conditioner is a product designed to clean the hair like shampoo while conditioning the hair like your regular conditioner. In general a shampoo contain negatively charged surfactants that cleanse while conditioners contain positively charged ingredients to attach to the hair to give it the conditioning properties. If the two are not combined properly, you end up with a not so great product.
So chemists have found a way to combine the two by hiding the conditioning ingredients in an emulsion that releases them when rinsing the hair. What actually happens is you shampoo your hair and everything breaks open so the conditioning ingredients such as silicones or polymers are deposited on the hair.
The advantages of using a 2 - in - 1 is you are saving time because you don't have to use a shampoo and a conditioner separately and money since you only buy one product. It also saves space in your cupboard, shower, or gym bag, and it promotes sustainability.
Unfortunately, 2 - in - 1's have some built in downsides. For instance, product buildup can happen faster than when using two separate products. The buildup can make your hair seem dull and limp. To counter this, you'll need to use a clarifying shampoo to strip away the buildup. In addition, this type of product can be more drying for many folks. This happens because the conditioning ingredients are not enough to counter the shampoo removing the oil from the scalp and hair.
Trying to find a 2 - in - 1 that works for your hair is going to take time and some trial and error. If you find one that promises everything, it won't. Furthermore, if you have curly or coily hair, this type of product is not recommended but if your hair doesn't need a lot of work, it will work for you. No matter what, one should not use this more than twice a week.
So the bottom line is that they are ok for some folks but not for everyone. Let me know what you think, I'd love to hear. Have a great day.