Does anyone use more than one robo-advisor(s) for diversification? Is this advisable?

I use two robo-advisors for different portfolios due to the underlying allocation. One income, one growth. Also to try out the platforms.

Not so much for diversification.
Since most robo-advisors invest into ETFs, it’s all “equities”.

Risk will be related to your risk appetite and the underlying ETFs/investments made by the robo-advisor. If the portfolio allocation between the robo-advisors are similar, there might be small differences in the returns. Again it also depends on the algorithms, rebalancing and investment strategy used by the robo-advisor.

Perhaps another way to look at this is understanding how to reduce your investment costs as much as possible to maximise your returns? This is something that you can do research on and control.