THere are several types of 2pc. Poser typers nothing joining the jacket and pants, good for Tim's,, cruising types with a one foot draft zipper on the back, comfy types with a 360 zipper sewn into spandex. and a track type with a good zipper full 360 sewn to leather.
So the poser one is no good for track,the cruising type,or the comfy type. Some organizations don"t really understand the differance so you may sneak thru.
But the quality day provders won"t allow it. Plus you would be stupid to use one of the first three and have you jacket up under your armpits sliding down the track.
Don"T go and by comfy pants and then come to me and ask me to sew the zipper in for your jacket. If they don"t come as a set it won"t happen for cheap. So a good track two piece zipped up will not fit much different then a 1 piece. Lets use a Spyke two piece as example. Great track suit steel zipper and designed for the bad day.
It will have more bulk in the middle becasue the two overlap.So the jacket overlaps the pants. But allows you options as it is made for street and track so it's accomodating.
A good one piece allows you more flexability. A good one piece fits you on the bike,,not standing at Timmies. That is why most racer/track day guys unzip the suit while they are not riding. If you crash in a 2 piece and slid on your wasit/hip area,, expect to pay more money for repairs.
And altering a 2 piece,,,, forget it.
So to refresh,, 2piece zippers must be sewn to leather not the liner or spandex
And bought as a suit,
Designed for the track with solid zippers.
If your zipper fails trackside, slim chance anybody would stock the zipper you need to make it work. So your day is done.
Sucks becasue you spent how much money and time for a day with seven 20 minute sessions.