Running Excel addin for MDS2019 vs MDS2016
It's not entirely clear what part of the stack your question is on. Assuming you're running a second server in parallel with SQL Server 2019 with MDS 2019, then you can certainly connect to either instance in parallel. Specifically here's the download for the Excel Add-In for MDS 2019 which should be backwards compatible to connecting to your MDS 2016 instance. You should be able to even deploy your models from your 2016 instance over to your 2019 instance by following these docs.
If you do need to still give your end users a version of Excel with the MDS 2016 Add-In, then you'll need a separate machine (a VM, or however you prefer) with Excel installed and the MDS 2016 Add-In installed, as I doubt there's stability in installing two instances of Excel, each with the respective version of the Add-In, on the same machine. But maybe it's possible, you'd have to test first.