You need to have an associative table ( create it from your Parse.com website account ) that should contains the pointer to the sender User, the pointer to the Receiver User and the message to send. So:
let's call the table class you need as "UsersChat"
objectId
(string) default parse field
senderUser
(Pointer:User) pointer to the user who send the message
receiverUser
(Pointer:User) pointer to the user who will receive the message
message
(string) concrete message to send
createdAt
(Date) default parse field that contains the creation row, so the sent message date/time we can say
updatedAt
(Date) default parse field
ACL
(ACL) default parse field