ShortTermRental + 9-5 W2 Job. Can I claim Home Office as primary?

[Mike H.]

Hello.

I have a duplex where I live on one side, and rent the other side on short term rental (considered as an active business for the IRS). 
I also have a regular full time 5days/week job on W2 at company X (single office, no secondary/client locations). 

I am interested in deducting the miles driven for my W2 job. For this to work, I would need to claim my home office as primary.
How is that decided?

Is it based on income? Incomes are pretty much equivalent. I can probably make it such that W2 is slightly lower income.
Or is it based on time spent? I work more hours at my W2 job, so unfortunately that wouldn't beneficial for me. 

Or is it based on availability? The STR property requires me to be available 24/7 with prompt response time, so that could work.

Thank you!