I need to connect two or more Heroku Connect add-ons to a single Salesforce Org, with the same mappings.
You can set up multiple Heroku Connect connections to a single Salesforce Org, as long as each connection has a unique database schema. So
app-a::DATABASE ::salesforce ,
app-a::DATABASE ::another-schema are all fine but you couldn't have two connections in
There should be no issues regarding performance with a setup like this, as long as you are not trying to sync two extremely large connections at the same time on a very frequent basis. Eg: if you had hundreds of millions of rows being updated millions of times per hour by both connections; this could cause performance issues.