One more thing I just now thought of. If you can handle more than two new chicks, it would really be better for them if they were a larger number, like four at the minimum. If you can handle six more, that would be ideal, and trust me, you'll probably end up wanting more by this time next year anyway, so may as well just go for it now!
The reason behind this is that your six five-week olds are a unit. They will be BFF's for life, and will do everything together. This is how chickens roll. So just getting two new chicks is going to put them at a distinct disadvantage as a unit of only two. If the new unit were to be four, or ideally six, they will be less likely to be treated roughly by the original six.
My youngest three are only just barely a large enough unit, and they get constantly pecked by the four-year olds, and the three-year olds, as well as the one-year olds. They are the smallest BFF unit in the flock, and I think it would have been a bit easier on them if I'd made it four when I got them.
You've heard of "chicken math"? Well, this is a variation of it, and by increasing your new batch to four or more will give you a big leg up on chicken math, which has already claimed you as a new victim. May as well just surrender and go for it!