Connect LinkedIn Lead Accelerator to multiple instances?

This is only somewhat true. LinkedIn transitioned Lead Accelerator into their self serve Campaign Manager.

However, the core Lead Accelerator functionality still exists, just with a different name: LinkedIn Account Targeting.

You can follow these instructions to connect the new LinkedIn Account Targeting just as you would Lead Accelerator, with a few notable differences. Hopefully, Marketo will update their nomenclature on this soon enough.

And to answer @Stephen Jeong​'s query, yes, you should be able to use Launchpoint to create a LinkedIn connection to multiple LinkedIn accounts, and/or to multiple Marketo instances.