Replacing DVD Drive with HDD/SSD

You asked:

My Question is, if i press the eject button on the keyboard, What will happen?

Nothing will happen. The second HD/SSD is an internal drive and internal drive can't be ejected.

To elaborate on Illaass' comment, the Mac detects it is an internal hard disk or SSD and knows it cannot be ejected. If it was connected via an external connection or what the Mac thinks is an external connection then it could be ejected. Of course this will not mean it will be spat out the DVD slot! It will however in that situation disappear from the desktop.

This later situation only really applies to USB, FireWire or Thunderbolt connected drives, or specifically to the old classic Mac Pro. With the classic Mac Pro it is possible to fit an internal PCIe SATA card and even though this is the same type of interface i.e. SATA the Mac knows it is not the standard built-in one and treats it as an 'external' connection. There is a free third-party KEXT driver which overrides this.

In any case as far your situation is concerned the Mac will treat this second drive as a standard internal drive and you will not be able to accidentally eject it. (I have fitted one of these previously in a MacBook Pro.)