Can both the Origin X1 & Rover dock securely inside an Origin 600 at the same time?

This should certainly be possible yes. The lift platform itself is the perfect fit for the Rover, but there is nothing to stop you storing the X1 in the cargo storage area of the exploration module. Bear in mind that if you choose to do this then it wont be a quickly accessible option and as such we'd recommend a Rover or X1, rather than both.