I read this question (Do mltiple (.NET) sites in same application pool share a JVM?) and it confirms that two applications can use the same APP Pool which will make both applications share the JVM. Now, apparently, SDL advises not to do so especially due to conflicts with Ambient Data Framework (ADF). How about having two deployers? AFAIK they don't use ADF.
No, it is not OK - unless they use the exact same configuration files, including storage locations.
I've done this before by mistake, and only noticed what happened when I saw XPM tags in the live website. Basically, whichever deployer starts first will load the configuration into memory, and this stays on.
If you have different configurations - which you certainly have for staging and live - then they cannot be in the same application pool.