Hm. It is still not clear from your description that the heartbeats and data transfers take place concurrently.

1. Does a test result data transfer take longer than 10 seconds?
2. Do you actually see a heartbeat get sent from client to server, whilst the a data transfer is in progress from server to client?

   (I'm not sure that this is even possible!)

3. If so, what happens about the servers reply to the heartbeat?

   Does it suspend the data transfer in order to send the reply?

   Or does the reply get delayed until after the transfer completes?

   Or does the server ignore heartbeats sent whilst a data transfer is in progress?

I suspect that the logic is more likely, the client sends a heartbeat every 10 seconds unless a data transfer is currently in progress.

If that is the case, the requirements are actually quite simple. But as you've described so far, I don't think they are possible, using threads processes or a select loop.