By speaking the same language : ICC.
And they can be speaking past each other, or speaking in different locations if you'll beg the analogy. Yes, the display profile and printer profile are 'linked' so to speak, when you soft proof. The creation of the printer profile has absolutely no knowledge of the display when built nor does the display know anything about the printer. Further, these printer profiles have two tables, one that affects output, one that affects the soft proof. In a prefect world, they are in sync and not only speaking the same language but understanding the nuances of the language. But that may not be the case. The printer's soft proof table requires a properly built display profile as well, one that is built to create a visual match to the print which means the print viewing conditions are part of the conversation. If the display is calibrated and profiled and this important part of the conversation isn't discussed, there's a mismatch.