Haven't tested it with Opal drives but with software encryption MDE will identify the 2nd drive and will encrypt it as well.
It is important to know whether the 2nd drive is Opal or not. If it is and compliant with MDE 7.1.1 most chances are that you will be just fine and both drives will be Opal activated.
However, if the 2nd drive is not Opal compliant, I would recommend deactivating MDE (disabling Opal activation on the first drive) before inserting the 2nd drive and then re-activating when both drives are in the system.
Opal encryption requires all drives to be Opal compliant and if you add another drive which is not Opal compliant to a systems which is already Opal activated you might see some funky behavior on MDE side.
The best option would be to test it on a test system, but if you don't have that option, I'd prefer being safe than sorry.
If you drives are Intel made, then MDE currently supports Intel ProSSD 1500 or Intel ProSSD 2500 models for Opal activation.
Good luck !