Was this second bag a Mercari purchase? Because I had this happen to me too. Four days and nothing, I sent the seller a message and they asked me if I wanted to cancel. No! Send me my item! Seller didn't reply to that message so I contacted Mercari and they went "well, we can prod the seller or you can cancel." I say prod, I assume they did, still nothing so I contact Mercari AGAIN. Item finally ships almost two weeks after I bought it, but I did get it. Worst part? It didn't let me give the seller under a four star rating, because the item was in good condition as described.A purple kimball - seller mixed up the packages and sent me a brown small sidepack (I don’t know what the style name is for it) instead. She told me she’s been trying to message the other buyer to return it but they haven’t been responding and the transaction is about to close, which means the other buyer can just keep it, it seems. The other one is a gray collegiate - it’s past the 3 days in which the seller has to ship the item and I haven’t heard from them. There’s still a chance I can get these two bags but I’m not really going to put any hope into it.
That being said, I got a really good deal on the item so I'm honestly thinking the seller was hoping I would cancel so they could resell for a better price. Well, too bad!
Tldr, don't be afraid to contact Mercari... even more than once.