You can you try next getRequest:
NSMutableDictionary *getRequest = [NSMutableDictionary dictionary];
[getRequest setObject:@"created_at" forKey:@"sort_asc"];
NSArray *users = @[@(2000), @(1992)];
[getRequest setObject:users forKey:@"receiver[in]"];
[getRequest setObject:users forKey:@"sender[in]"];
[QBCustomObjects objectsWithClassName:@"Chat" extendedRequest:getRequest delegate:self];
In this case I get all messages between users with id 2000 and 1992
UPD: now it seems correct. (Try pass same users' id array in getRequest)