This one is pretty simple - go as wide as possible. Choose themes, which have as little in common as possible. Make sure every player will have at least one he/she will be interested in. If you have 2 escape rooms and both are horror-themed - you are going to cut off the significant amount of potential audience. Kids, families, people who hate everything scary - they will be completely negative on giving your new entertainment a try.
If you are on a small market, where people don't necessarily know about escape rooms, it will hurt to lose a potential customer just because both of your games themes are "hard pass" for them.
On the other hand - imagine a situation, when you have a horror-themed game and a kid-friendly one: almost every person will be ok will playing at least one. And once people tasted what escape rooms are - it will be much easier to convince them play another one, even if the theme is not so appealing to a certain person.