2 in 1 is a short sewn to a jersey around the back of your waist but the two pieces are separate in the front. It has pockets in the back (Castelli’s San Remo has 3). It has a full length zipper in the front. It’s basically a replacement for a bib+jersey. Aero advantages are not tremendous. It’s a direct replacement for longer rides when you need to carry stuff and may need to go #1 (and have male plumbing).
An aero skinsuit has the two pieces completely sewn together all the way around your waist. Typically no pockets, or just a pocket for a race number. Has a measurable aero advantage. For TT, crits and events where you will not need to carry anything in back or get off the bike.
So which type you should get depends on your goal. I like Castelli’s San Remo (2 in 1) but I’m biased since they are a team sponsor.
The main recommendation is that you find a shop that lets you try on before you buy and sit on a bike in it. An aero skinsuit (Castelli body paint) may feel uncomfortable when you pull it on because it’s designed to fit right when in the aero bars, typically.