I think it makes sense to invest with multiple robo advisors to take advantage of different portfolios. Depending on your interests, you will find that robo advisors hardly offer an exhaustive range of funds or ETFs, so it makes sense to use a couple. For example, ETFmatic only offers ETFs, True Potential Investor dabbles in active funds, and most of the others simply sell a combination of index funds.
If you decide that you want to use at least two robos, pay particular attention to fees. I would use robos that charge a percentage-based fee as opposed to a fixed fee. My reasoning is that with percentage-based fees, provided the fees of both robos are similar, you'll end up paying almost the same amount that you would have paid if you invested with just one robo. However, if you choose to go with fixed-fee robos, you might end up paying double. I am a massive fan of Koody's, and I have used their price comparison tables28 to compare fees. For robo advisors, you want to look at the third tab.