Double dildos can be a bit daunting, especially because they are so frequently referred to as "strapless", implying that they are to be used without a harness. While that is possible, double dildos are designed with a bulb end that penetrates the wearer as well as the phallus, or what we would typically picture as the dildo to penetrate their partner. That takes a lot of strength, practice, dexterity and, frankly, is just a lot more challenging.
I like pegging superstar Ruby Ryder's approach of referring to them simply as "doubles;" it takes away that expectation. Now, as far as good harnesses go, harnesses are often a matter of personal preference but my favorite recommendation is always the SpareParts Joque. It's comfortable (and keeps the wearing comfortable while using it), super-adjustable, looks great, and works with just about any dildo you throw at it.
Learn more about strap-on sex in A Beginner's Guide to Strap-On Sex.