I understand that you want to know if you can connect a second Slingbox PRO to the set top box.
As a matter of fcat, it is possible. It all depends on the available outputs on the back of your set top box. However, this connection will allow to watch the same channel on both Slingboxes. It is not possible to watch a different channel.
Is it possible to connect another Slingbox Pro on top of my existing one, to the set-top box? I have a direct connection to the cable - no network, etc.
Yes, and it doesn't actually have anything to do with the connectors on the settop box, just use the pass-through feature on the first Slingbox to drive the second Slingbox. I actually have my Sky box connected to 3 different Slingboxes, just daisy-changed together from a single output on the device.
Also, are 2 IR Blasters needed for each Slingbox, if I am able to stack the Slingboxes?
That question doesn't actually make sense as the IR Blasters are positioned in front of the settop box, not the Slingbox, so having them stacked is irrelvant, but I will try and answer it
First thing to note is that there are 2 IR Blaster heads so you can connect 2 different devices, so you only need one blaster to be positioned in front of the settop box. But you will need one from each of the Slingboxes, so that you can control the settop box from each one.