Q:

How can I push to Heroku, and still keep the web app live?

Say for example you had two web dynos set up for your account (0 worker dynos).
To save on switching to maintenance mode, how would one push to one web dyno, and then the other update once the first has finished booting?

A:

You can't do that - pushing to Heroku will result in both dynos being restarted with the contents of the new slug.
However, there is a labs feature called preboot (https://devcenter.heroku.com/articles/labs-preboot) which might accomplish exactly what you want.

