To "chat" or NAP - that is the question!

M&A are 15 month old toddlers who I care for during the day. They share the same room-cribs within arm's length of each other. They love to stay up during the day "chatting" instead of napping. If they don't nap they are close to non manageable by end of the day. They sleep well at night. When is it a good time to give M her own room - to separate them. I want to because i put them to bed most during the day but mom does not want to...