How would second charged moves impact the gym offence/defence metagame?

We don't know how defenders will work with the new move so it's hard to say. One thing is sure, togekiss and Gardevoir will be less useful as Machamp walls because he can carry HS as a second move.

Defenders is an interesting variable, that will be interesting to see how it is handled.

One thing that will be impacted (if they don't expand move pools per mon) is that CTMs will become less problematic to use.
That is, set up 2 moves, and then only a single CTM is necessary to get rid of the least desirable move (in most cases).
...just a game mechanic that I realised may be impacted I thought I would mention

The only way having double charged moves would help is if there is counter coverage (if you absolutely don't want to switch) Or if you're almost dead and want to fire off a Close Combat rather than a Dynamic Punch. (Using Machamp as an example, reason is one CC does more damage than one DP, and the Machamp may die after the charged move, assuming move time is fast)

I expect defenders work like raid bosses:
When the game is deciding whether the raid boss should next use a fast move or charge move, if it has enough energy to use its charge move, then there is a 50% chance it will use its charge move.

Extrapolating that for defenders with two charge moves, they are probably going to be more likely to use a 33-energy or 50-energy move before they save up enough energy for a 100-energy move.

If enough energy has been saved to use both charge moves (including of course if they have the same energy cost), I expect the game should be able to see which charge move would deal more damage and choose that move when it randomly decides to use a charge move.