I'd also check out what would happen if you wanted to go away for a 2 week holiday... At the moment it would just be 10 days of leave for 2 weeks off but that would actually include 3 weekends- but it would start to get complicated when factoring in having to sort out every other weekend working, particularly if there is only one other person you alternate with. It could mean that it becomes very difficult to take a fortnight's holiday leaving from your house early on a saturday (or sunday) and coming back late on the saturday/sunday 2 weeks later which is fairly common.
My dsis used to work somewhere she was mainly mon-fri but there was an element of having to have somebody in at the weekend so there was a rota (but not retail so not quite the same has having to frequently work weekends). Even though there were about 4 or 5 of them that did the job so they should only have to work the weekend once or twice a month, they had a really mean and spiteful boss who used to deliberately do the rota so that if one of them took 2 weeks holiday she would do it so they didn't get their 3 weekends around 2 working weeks - so they would either have to use extra holiday to get their 3rd weekend or not have quite such a long break. Most of the time the others would have been happy to change (they were all friends) but nasty boss wouldn't let them.
Even if you don't particularly want to take fortnight-long holidays now, you might at some point, as might your colleague, particularly if he is from a different country and might want to travel home for a slightly extended period (what would happen if say he had to go to his home country for 6 weeks - would you be expected to work every week in his absence?
What happens if you need to do more than 4 hours on the weekend? What happens if you do 4 hours on Saturday - will you then have to do more time on a sunday? Will they want to formalise it to a particular timeslot - which becomes quite limiting to your weekend potentially, particularly if you have other things that you or your family usually do (say football matches with the kids or cricket practice or dance lessons that are a weekly commitment at a time that can't be changed - but might of themselves change - say as kids get older, they move through the classes and go 'to the next one' so while it's still taking a chunk out of your saturday morning, it's from 10-11 instead of 9-10.
What happens if you're sick at the weekend?
Is 2 hours a reasonable amount of time to get done what they need to get done?
Can you log all the extra time that you and your team mate are doing - is there enough over the week (particularly when you factor in overtime and holidays) to argue that actually if you're going to do this you need to bring in an additional team member. Which would help by reducing weekends required to one in 3 and giving you back some of your own time that you're currently giving them in doing lots of unpaid overtime. It sounds like this is becoming more of a business issue in so far as it didn't used to be necessary and is becoming more and more so - so is likely to become more and more necessary so it could creep to half a day each at the weekend. THey might be hoping that you will do half a day each day at the weekend and they'll be getting an extra half day from you for free on top of the other overtime, plus be able to jerk you around more if you have formal times to be working at the weekend.
How time critical is stuff that you do - might they say that your hours are say 10-12 but then get pissed off because you didn't spot something that was critical or had consequences at 5 past noon as it was outside your hours - and what if you spotted something at 5 to noon and it needed 3 hours work but you had plans for the afternoon wtih the rest of your family (visiting friends, taking a child to a party, going to the theatre, travelling somewhere)
Will they always expect you to do it from a home office or could you say work from your parents' house or a hotel if you go away?
I think if you are doing a job that is normally mon-fri 9-5, even if you end up doing lots of overtime, it is a massive switch to suddenly give up every other weekend for a couple of hours a day.
Could you suggest switching the hours even more if it would work in your favour - so say 3 hours a day at the weekend/6 hours over the course of the weekend as required/6 hours as suits you best, then 1 day off in the week but 2 days with an extra hour tacked onto those days (so 2x 9 hour days if your normal day is 8 hours). If you are likely to end up doing longer at the weekend anyway, and you are likely to be doing overtime anyway, then at least this way you are carving a day off for yourself rather than a half a day off when you would be doing the work anyway... Or even push for the 4 hours at the weekend and a full day off, with two extended days as you do it anyway, and that the missing 2 hours are for compensation for having your weekends screwed up (they're not going to offer them to you but doesn't mean that you can't negotiate - bet your boss is not going to be giving his weekends up and if he does, he will be much better off for it than you).
Do you know what senior managers who are at a similar level to you do when doing weekend working when it's not been a normal thing for them - do they get a nice juicy on call bonus or double hours off in lieu or anything convenient that you can use to make things better for you?
What will happen if there's a problem? Will senior managers also be working or on call so that you can raise problems with them?
Sorry, lots of questions but no answers.
And will echo what a previous poster said - definitely worth checking out your home insurance to see if you have legal cover that could help you as a free and usually fairly speedy starting point!