The more costly one is to separate the meters which will be a costly upfront cost but your cash flow will be more predictable.

Eventually the owner got tired of the complaints, relented and had a separate HVAC unit installed.

A partnership is a marriage, if it breaks up, it is rarely amicably, and there is always pain, whether it be emotional, psychological, and/or financial.That being said, you need to talk to an attorney to create a well structured partnership agreement.  

I have zero emotional attachment to any property whose numbers don't work, and would happily move onto the next one.All in all, I wholeheartedly appreciate your advice.

We have separate company for our two SFH.

I’ve talked myself out of a couple good opportunities by listening to my emotions too much, and not trusting my the numbers even when they met my criteria.

Would we have to establish a separate LLC, making them a partner?

The purpose of this is to make it crystal clear who is responsible and to avoid a mess if they decide to go separate ways during the tenancy.

You can include it rent and just increase the rent (no need to separate it out fee wise just bulk it in). 

There is a house for sale that is rented out to a few separate small businesses (hair dressers, etc).