Let's hit this from an Ops tuning perspective... I do company turnarounds and start-up processes, hiring, and anything that does not require the big vision or heavy tech expertise. These usually start out full-time, then trickle down, as planned, to nada. So once you have the processes set and have taken the big actions, you should be able to run two firms at once.
Here are a few tips:
1. Do not share employees/founders between the two.
2. Do share vendors/outsource between the two.
What this means is keep your people focused on what they do for that one firm. Seek to use shared vendors for HR, marketing, advertising, and anything else that is not core to your business. You will save much time working with the same people.
Set recurring meetings with your various teams at each firm so that they all feel they are getting your dedicated attention. Have each Team or person setup a spreadsheet or some type of todo/project system entry that you and them jointly manage. We use Asana but even a Google spreadsheet will do. Have them (and you) put all topics each of you want to talk about when you meet, yes, this is a rolling agenda. Simple and complex questions go here... If you (or they) can answer a question, just do it in the file, no talking needed... The complex issues, people issues, strategizing, etc. can be done in person. Insist everyone write down their questions and other topics. Do not be immediately available except for real emergencies.
Ping me for more. This is not too much different than managing say, a tech team in Austin and an Ops team in Philly :)