Not two regular bags of holding, since that is not described in their rules. If a DM wants such a thing to be true though for their story and their universe, then sure thing. It'd be quite the cool mechanic. That pair of bags would be much rarer and far more valuable than other bags of holding though.
Sort of like the Spell [Demiplane] then? Accessible if you know the place, item, etc?
Yeah, but I would require some very detailed Arcane knowledge to connect a new bag to an existing bag (Demiplane can't connect to a bag per RAW, only to other Demiplane spell planes). Essentially we are talking about crazy high level magic here, so it would be something the DM provides for the game, rather than something the players would be able to craft or create. Still, linked planes and portals and bags provide a lot of very interesting situations for higher level players to get themselves into and out of trouble.
Going off this point, what if the players happen on one of these items, and thinks of it as a standard BoH. Only instead it is the bag of sharing. The mate of which is held by an antagonist, say a lich, who uses it to remove things from the party from time to time. It mean it may seem like a petty thing to do for a super-genius lich to pilfer items from the party, but wait. If he can manipulate the situation so that the item falls into their hands he can in theory deprive them of needed items at his leisure. Chasm? Bet they would like some rope. ZOINK! On the other hand it would also be used to further it's plots by providing items into the bag that the PCs happen across. Funny? When did we find this red box?
Can two bags of holding be linked to the same nondimensional space?
Not two regular bags of holding, since that is not described in their rules. If a DM wants such a thing to be true though for their story and their universe, then sure thing. It'd be quite the cool mechanic. That pair of bags would be much rarer and far more valuable than other bags of holding though.
Sort of like the Spell [Demiplane] then? Accessible if you know the place, item, etc?
The better question here is:
Do you want the bags to be able to share the space?
Yes? Then yes, they can.
No? If you don't want them to be able to, then they can't.
They function differently than your usual bags of holding though, so you might need to rename it.
Going off this point, what if the players happen on one of these items, and thinks of it as a standard BoH. Only instead it is the bag of sharing. The mate of which is held by an antagonist, say a lich, who uses it to remove things from the party from time to time. It mean it may seem like a petty thing to do for a super-genius lich to pilfer items from the party, but wait. If he can manipulate the situation so that the item falls into their hands he can in theory deprive them of needed items at his leisure. Chasm? Bet they would like some rope. ZOINK! On the other hand it would also be used to further it's plots by providing items into the bag that the PCs happen across. Funny? When did we find this red box?