In BL70x series chips, the default JTAG gpio is GPIO0->TMS, GPIO1->TDI, GPIO2->TCK, GPIO9->TDO;
we can know this by the BL706 AVB schematic of the competition board.
Through the sipeed/RV-Debugger-BL702 open source repository on GitHub we can get the schematic of the RV-debugger Plus and we can see that GPIO9 is used to drive the DS2 LED; so if you want to debug another RV-debugger plus with one RV-debugger plus, then you need to pin GPIO9 out and use it separately.
Next you need to press and hold the boot button of the target and then connect the power, at this time GPIO0, 1, 2, 9, will default to JTAG mode, so you can debug it. Just like the following picture, have fun!