I'm in the process of starting up a new company, while ramping up an existing one. Are there any generally accepted ways to manage / document the splitting up of office rent, equipment, etc? In the short term I will be the sole employee of both, so I'm trying to find out what other people are doing regarding the bookkeeping in similar situations.
FWIW, the reason that they are two companies is that they do completely different things, and I'd like to keep them separate.
If you're the only employee, then I'd look at how you're spending your time. If you have an even split, allocate 50% to both. If you're spending most of your time on one of the companies, possible a 70/30 split would be fair.
I've never dealt with this in the case of on one person working on two companies, however, when doing cost account between departments in a company it is very common to allocate based on the number of employees or the comparative revenue. Unfortunately, it sounds like neither of those apply in this case.
For office space, you should split it.
As for salary costs, that's probably best done on an hourly basis or percentage of your time.
One thing you should do is write down how you did it so when more people get involved, it's clear how you did the books.