The question here is, are you prepared to pay extra to sit together?

When checking in, the software will most likely seat you apart. Ryanair starts seat assignments with middle seats, so if you check in early, you will likely get middle seats in different rows with seats next to you still vacant. After the check in, you should be able to change one of your seats for a fee, so that you would sit together. In my experience, this fee is identical to what you pay when reserving seats in advance.

If you reserve seats in advance instead, you would have to pay not for one, but for both seat reservations.

If you are not willing to pay extra to sit together, then checking in as late as possible (but before the 2h deadline) is still a better choice, because you would get better seats this way.