Wheel side quick release.
Your Simucube 2 comes with one of these, if you want to use multiple steering wheels you will need this piece attached to your rims.
 
Precision machined full-metal Simucube Quick Release guarantees zero backlash and rock solid operation even under the extreme torques of direct drive wheel base. Guided rail design ensures perfect lock-in every time, and spring preloaded pin mechanism will ensure non-degrading tight coupling over thousands of swap cycles. Just like rest of Simucube,  SQR has been built to be virtually immortal. The only practically wearing part, spring loaded pin mechanism, is made out of standard stock parts making it user serviceable.