Subscribe to two different type-not synchronized topics

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Hi there, I want to check two topics at the same time, but as the title says, they are different type and arent synchronized, so I think I cant use the message filter example in the ROS Wiki. Here is the relevant part of the code, where I subscribe only to the camera topic:

Inside the main:

image_transport::ImageTransport it(nh);
image_transport::Subscriber sub = it.subscribe("/ardrone/image_raw", 1, imageCallback);

Callback:

void imageCallback(const sensor_msgs::ImageConstPtr& msg)
{

I want to subscribe to /ardrone/image_raw and /ardrone/navdata topics. Image raw is type image transport and navdata is a custom type. Also, an acceptable option would be that the callback only response to the Navdata topic, but inside i could read the actual image in the image_raw topic, cause the navdata topic is faster as the camera topic

Thanks!