Before I was on the unlimited plan I had the hotspot SIM added to my main plan as a data sharer, but unfortunately it seems that the unlimited plan does not currently allow more than one SIM.
Staff from the Telco's can't talk about future plans or changes until it's public information and generally up on the website
What your asking for is a Share plan.
Currently Share plans are not supported on the Unlimited service.
You can switch out to the 7.5GB plan for the same price, this may not suite you though.
Any comments made are personal opinion and do not reflect directly on the position my current or past employers may have.
I'm not sure any telco would ever add a shared plan feature to an unlimited plan. It just doesn't make sense.