There are reasons to have more than one, but whether they’re good reasons depends on individual’s scenarios and needs.
For example, I can’t take my work pension before 65 without taking a big reduction - so for me it makes sense to save into a personal one with the aim that I use that to retire 2 years before 65.
But in spreading the risk…
Your work pension will not be invested in one company’s shares (which would be very risky). Generally you invest in a fund, and that fund is a mix of different types of investments - different companies, industries, geographies, some government bonds (different to shares). So within the fund you have now, there should be a spread of risk. In addition to that, the fund is probably “managed” meaning that the fund provider is rebalancing it.
If you don’t mean the investment risk but the risk that a pension provider will go bust, it’s not the same situation as you may have heard of years about, with regards to final salary pensions. Today, there is sone protection for these from the Pension Protection Fund, and in any case you are likely to have a different type of pension. If your pension provider goes bust, the fund they invested in for you would still exist. So also no need to manage risk there. The fund itself can still go up and down - but see my previous paragraph - a fund won’t have all eggs in one basket anyway.
Start by finding out what fund your work pension is invested it - if it’s not final salary / CARE.