Hotspotting just provides a gateway for a second computer, phone, or tablet to connect to the internet. As far as the phone system is concerned it is the same as having two, three, or four windows open on a screen at the same time.
If you have a fixed time connection of 60 minutes, you don't use the time up twice as fast if you have a second device hotspotted to the first one. Both devices can be used during that 60 minutes.
If you have a fixed data allowance, then the data allowance will be used up more quickly with two devices connected because each device will be sucking data from the internet.
The hotspot data transfer is via wifi or bluetooth, so that doesn't count in the calculations, only the data up and downloaded by both devices from the internet, or the total connection time of the device with the SIM card. So you won't use any extra data using one iPad as the hotspot while the other one is actually doing the internet searching, or whatever.
Hope that helps clear things up a bit.