Can you physically put them in different rooms?

My day care is on my main level and the under 2's nap in my guest room and the over 2's nap in my room. When I've had this issue, I've been fortunate to be able to put up a crib in my walk-in, so I would put one in there and completely separate the talkers.

If that's not an option, the two ideas (which you've likely tried already).

1. When you hear the chatting, go in and lay the both down, and shhhh them before walking out. This might need repeating a few times before they get the message.

2. Can you get them up? I don't leave mine in their cribs until end of quiet time. Some need less sleep than others. One of my youngest is 16 months as she's always awake long before the others but the older ones have had lots of running, walking, climbing and are physically exhausted. Where she is less steady, she ride the wagon for much of our walking time, so she's not as worn out. She's a loud personality, so I scoop her right up so she doesn't wake the others, and she will play independently with puzzles or blocks as I prep for the afternoon. As long as I chat to her as I'm prepping, she's quite content to play on her own for the rest of quiet time.

Just one more comment. You mentioned that sometimes you tell the to stop talking and other times you ignore it. - I think the inconsistency might not be helping. If they are only going to be told to stop sometimes. then when they do wake, it's worth them chatting and carrying on, just in case today is a day you are going to ignore it.