Can a local user and network user use the same home folder?

I wouldn't do that, as I'd expect the file-tracking mechanisms — the network home synchronization — would get tangled.

You can do it if your home directory really is a networked home directory (e.g. served from an AFP server on the network). In that way both the iMac and the MBA can mount the volume and access your home directory.

The tech note you referenced does, indeed, talk about issues if the home directory is local on the server and you log in on that server.

Make two home directories on your server.

The first is your server admin account, on the boot volume of the server. You will use this when you're sat at the server because, for example, you're trying to figure out why Open Directory isn't working. Give this account an unusual name like Trops Bar, preferably a name nobody could guess.

The second is your Open Directory account which should be set up with a normal form of your name just like all your other open directory accounts, except that you give it administration privilages. It's in the OD database.

Use one when you're sat at the server and the other in normal life when you're sat at your office computer or anywhere else that uses OD accounts.