Any army, which is attacked, loses its support hold order, ALWAYS!
If both are attacked, both lose support.

So it is a good way of addressing the odds, but not bulletproof?

gameID=84842
check Autumn, 1902

If austria had attacked tyrolia from vienna, it would have taken away the support hold from venice, and he could have got venice successfully. It was totally useless to hold trieste, but he chose to do.
Addressing the odds is always depending from the situation, sometimes it's far more effective to attack possible supportert then holding.
In the example above, both venice and tyrolia would have been attacked from 3rd parties, so nor venice neither tyrolia would have been able to support eachother against trieste.

Support holding mutually is not a good solution always. Sometimes it's better to attack with both to the very same province, where a possible support comes form.

For examlpe both venice and tyrolia attacks trieste, than venice is defended, so is tyrolia, even if vienna attacks tyrolia. It's far more better way of addressing the odds, than holding venice from tyrolia.

Thank rokakoma. I missed you post, but thanks for your help.