Thanks so much

Naples would support Rome, even though Rome supports Naples. Both orders would succeed.

However, it's strictly better to have Rome support a move to Venice. Naples could still support Rome regardless.

Thanks @Claesar

I was thinking Rome and Naples.

Then what would happen if Rome was attacked by Tuscany and Tyrrhenian Sea? I presume Naples supports Rome, even though Rome is supporting Naples?

More of a curiosity.

For instance, in your only current game I can't see any of your units this question would apply to. There's no two units I would order to support Hold each other.

Yes, that's a legal move.

It's rarely wise to do so (I personally dislike units that just hold in place as it won't gain you any opportunities).

Another trick you could do is a self-bounce. You order both of them to the same adjacent province. That way, neither moves and both stay in position. However, this can be disrupted if another player suspects the trick and supports either unit into tge target province.

Let's say I have two adjacent units. If I don't want to move either of them, is it wise to support hold each other?

Or do you need to pick which one is support?