Earlier quoted context omitted.
I don't know why you'd think it'd be several hundred more years before communication could be done. If they can both observe each other, then all that's left is to devise a way to signal back visually. Seeing proof of one's neighbors would definitely drive people to develop ways to communicate, though I guess both planets would need to be similarly driven in order to establish communication.
Galileo was observing Venus in 1610; his telescope was enough to see the phases of Venus but probably not high resolution or quality enough to see someone waving Semaphore flags or lighting a bonfire. Astronauts in Low Earth Orbit can't see the Great Wall of China with the naked eye. What scale of 'device a way to signal back visually' could done with 1600's era manufacturing and technology?
I'll admit, it'd take a lot of effort/money to communicate and it'd be pretty slow, but it's not impossible to happen just decades (or perhaps even within a decade) of when its figured out that they had neighbors. Not hundreds of years.