I'm in level 3 history, but we're required to take 2 4th year seminars in year 3. I checked the ones I have been chosen for today and saw that they are both in second semester .

Having two 4th year seminars in one semester seems nearly impossible in terms of workload .. is there some kind of rule against this? I've emailed the professor in charge of the seminar selections, but is there anything else I can do to try and get this changed or am I ultimately stuck doing 2 huge courses in one semester?

if they're required then everyone should be doing them right?

and 4th year seminars probably aren't as difficult as you make them out to be

I checked the FAQ about seminar ballots and it says that these class lists are only tentative and they may change without notice, so there's still a chance that your seminars will change
it also says that it's difficult to switch out of one and into another without permission, but hopefully given your case they will grant you permission
if all else fails, there is one seminar offered this summer, that maybe you could take instead

I took 2 fourth year seminars this year (each a full year) and it wasn't that bad. I'm in Political Science, but a friend in Pol Sci and History combined honours said he is taking a history seminar as well, and they are about the same.

Seminars don't add too much to your workload. I just see them as any other class basically. I'd say you're lucky that they are only one term long

I found my fourth year seminars to be easier than my third year courses. More reading was required for the one, but the work load was lighter. The one we just wrote two essays and the exam was multiple choice, and for the other seminar we just wrote one paper, did one presentation, and had a midterm. These were for Sociology.

You can sometimes find old course outlines, which will give you an idea of what the course load will look like. I'd check them out before worrying about how much work they'll be.