Let's do the maths here...
You say they want you to do these overnight double-shifts 6-8 times a month. Working from 17.30 to 9.30 with a 2-hour break adds up to 14 hours of work. If the shifts do count as 2 days, then that's an average of 7 hours "per day" for 16 days.
The normal shift is 9.00 to 17.30 - assuming a typical 30 minute lunch break, that is 8 hours per day. So, you're doing one less hour "per day" on the night shifts - or 16 hours less each month.
Sounds great, doesn't it?
Except - those 2-hour breaks in the middle of the night shift are in the middle of the night. What can you do for two hours in the middle of the night?
On the other hand, those double shifts sounds like it means mean you do 16 days of work in only 8 days - so you have more whole days without being rostered than you would if your did the same hours of normal day shifts, don't you? This is why some people work 10-hour days for a 40-hour week - they get a 3-day weekend. With this shift system, you're essentially getting 4-day weekends, aren't you?
As pointed out in a comment by puck - this isn't quite that straightforward. Sure, you're doing 14 hours work in a 24-hour period - but you're actually working across two days (starting on Monday evening and finishing on Tuesday morning, for example). Say you do four of these shifts in a block - your first shift starts on Monday evening, and you finish your last shift Friday morning - you're going to want to sleep Friday and some of Monday. Then again, doing two of these blocks in a fortnight means you only have to do 6 day shifts throughout the rest of the month (instead of 10 or 11 days). But this also means you're only getting 8 hours to get home from your Monday/Tuesday shift, eat dinner, sleep, eat breakfast, do other things, and then start your Tuesday/Wednesday shift. There may also be rules or laws around the maximum number of hours you can regularly work in a week - meaning you'll be mixing up night shifts, day shifts, and rostered time off.
Personally, I wouldn't take a job with such a schedule - constantly swinging between day and night shifts will seriously disrupt your sleeping patterns, and studies have shown it to be detrimental to your physical and mental health in the long run.