So what you need to do to make Trends work is just have one side of the transaction categorised as savings. Otherwise, as you say, they’ll cancel each other out.
For me, I use my Monzo current account as the “hub” for all savings. I categorise any outgoing payments as savings and make sure that the incoming transactions are categorised as Transfers. That’s the default behaviour, I think, of the Monzo savings pots and it works well.