It's really up to you!
Any of the strap-less strap-ons we have which are less than two inches in diameter (which, I think, is all of them) will fit into the Joque. I would recommend using a dildo that is two-sided as using two separate ones may cause issues when the bases are rubbing against each other (just awkwardness issues; no physical ones).
Some of the neat double-ended dildos we sell include the Feeldoe
line and the Share
line. We've used double-ended dildos with our Joque with no issue; it actually helps hold and support the dildo really nicely!