The thing is unless I've misunderstood the question neither of your employers is asking you to work 7 days a week - the "regular" job is asking you to work 5 days (which is pretty standard) and the part time one is asking you to work 2 days. So the question of "working 7 days" is something of a red herring here. I'm not trying to be insensitive - believe me working 7 days a week can be awful and I'm fully sympathetic to that, it's just not technically what either employer is asking you to do.
The real issue at hand is whether your part-time job can require you to work two days instead of one and to answer that will depend upon your employment contract with them (which should detail hours of work and any scope for changing them) and any local legislation surrounding change of working hours in an existing job.
As to whether they can "force" you to do the two days - well ultimately no but you may not be in a position to "force" them to continue giving you the existing one day either and that "no" may translate as "no, because you can resign".
A few more thoughts following updates from the OP:
There is a change coming in Ontario legislation that might apply:
In particular, employees can refuse shifts without detriment if they are asked to work with less than four days’ notice.
Unfortunately that isn't coming in to effect until January 2019 so that probably doesn't help you here.
If the day in dispute is a Sunday (which it sounds like it is) then that's a bit complicated but I believe that you will have the right to refuse to do so as long as you didn't agree to working Sundays when you were hired and that being the case they can't penalize you for refusing either.