Use Flex as receiver ( RX )

OK as long as you use a Transmit/Receive switch. You don't want to introduce RF into the receiver input stages. Sure death for the receiver.

Yes, or an RTR-2.  Take a look at the RTR-2 User Guide.  It has many diagrams showing possible configurations:

https://www.dxengineering.com/parts/dxe-rtr-2/documentation

I have used the previous model (RTR-1A) for years with my Flex and other radios with never a problem.

Howard

I can send you the RTR2 guide if you can't access the DXE web site.  It has detailed diagrams for many configurations and should answer all your basic questions.  It is very useful even if you go with the MFJ or another brand. My Email address is good on QRZ.
Its all very simple, connect the switch to the antenna, to your transmitting rig's TX port, a Flex antenna port, and use either PTT or RF sense (MJF unit only) for switching (I would use PTT if you select the MFJ).  Just remember not to transmit with the radio used for RX only!

Howard

Rui, pse send me email to rn3kk.nick@gmai.com I have info for you