It is possible that both roots may have fractures. This would depend on how well you remember the impact with the tea cup, whether it was just on one tooth, or evenly across both teeth.
The thing about root canal treated teeth is that once the nerve and blood supply is removed, the tooth structure becomes more and more dessicated over time. And the root as well as the tooth structure above the gum line (and/or under the restored crown) becomes more and more brittle.
So even if the impact didn't feel all that significant when it occurred, if it was a sudden impact, it would have been more than enough to produce stress fractures.
As to your question of whether you'll be in constant pain during this next 6-8 months before we take another X-ray to see if we can observe radiographic changes, the answer would unfortunately be yes. Somedays the pain will be greater than others. Other days, it will likely just be a dull, low-grade throbbing ache
that is manageable with pain medication. However, you could also follow-up with your general dentist in between to have him/her measure your gum pockets around these 2 teeth. Generally, if the root fractures were vertical, and located cervically (as opposed to apically), then sometimes, a vertical perio pocket can be detected with the perio probe. We could see the normal 2-3mm as we walk along the gumline and then suddenly, we'll see a 5 or greater pocket in one spot. However, if the fracture is horizontal in nature, then we will have to give the process time to cause decalicification of the bone so we can visualize it on the X-ray. I guess these are the things that test men's souls.
The fact that both teeth were root canal treated, plus the incident with the tea cup, and assuming the onset of pain coincided with that event (or very shortly after the impact), the most likely scenario is the presence of root fractures.
I certainly would like to be proven wrong in this case, which is why I would rather that you endure some pain and wait it out, than to doom the 2 front teeth to extraction. And since I do not have the opportunity to examine your teeth, as well as review X-rays, at best, XXXXX XXXXX merely speculating here.
But just to cover all basis, I would encourage you to explore restorative options now, so that IF we ever have to cross that bridge, you'll at least have had some time to think over your options to decide the course of action.
Best of luck to you.
JulXXXXX XXXXX, DDS