Seeing and hearing of the Messenger ﷺ, or the awliya, even if it's witnessing us in their graves and hearing our calls, with our belief in tawhīd and as explained above isn't shirk, even if many people are calling upon the righteous many times at once even from afar.
Remember, the attribute of Allāh is all hearing, NOT hearing from a physical distance.
We humans that call out to the Messenger ﷺ or the awliya will always be in a place, in a physical dimension, so even at the far edges of space or on the moon, wherever it may be, (SINCE WE WILL STILL BE IN HIS CREATION I.E. A PHYSICAL PLACE), it is possible for the Messenger ﷺ or awliya to hear us by the permission of Allāh (this is true affirmation of Tawhīd!), so this doesn't mean one is attributing divine abilities to the creation since it isn't a divine ability for Allāh to hear from a physical location being faraway, since Allāh is transcendent beyond His own created creation, dimensions and limits He places on His own creation. Likewise, it is possible for multiple calls at one time, whether that be a million or a billion calls, to reach the e.g. the Messenger ﷺ at one time by the permission of Allāh (we can't imagine how such information will be processed), but this doesn't mean we are associating partners with Allāh, since Allāh isn't only limited to hearing a limited number of calls, rather the one who says that we have associated partners with Allāh on account of this, are they themselves limited Allāh by saying if we say creation hearing limited amount of calls is equal to Allāh's hearing, since we know the hearing of Allāh is an eternal attribute that hears all, all the time, and isn't limited.
Yes ofcourse, if the Shi'ah say, we believe Sayyidunā 'Alī has independent hearing it's shirk. Or if they say, he has eternal attribute of hearing it's shirk. Or if they say, he had unlimited hearing, it's shirk.
And Allāh knows best.