Exit journey when entering another journey - how to set up?

You don't need to create a new field in Sales Cloud for this.

You can do it this way:

1. Create a new (sendable) data extension that would record in which journey the contact is currently

2. Use Data Designer in Contact Builder to either create a new contact group or edit an existing one

3. Link you data extension to the Contact Key - this will make the data extension available as part of Contact Data

4. In your journeys add Update Contact Blocks and configure it like this at the minimum: The SubscriberKey (or however you want to call it) needs to be dynamic for each of the people entering it and the way to get that in this block is by using data binding - you need to put this in the field: {{Contact.Attribute."Contact"."Contact Key"}}

5. Use this data extension as exit criteria, filters or decision splits in your other journeys

I described building a basic journey log here.