This scenario only occurs when XSDB disconnects from the processor before the Linux kernel has booted into full SMP mode.
When booting using petalinux-boot --jtag to boot all the way into Linux, the PetaLinux tools handle this connectivity automatically.
When using petalinux-boot --jtag to boot into U-Boot and then subsequently boot into Linux, you should keep XSDB connected to the processor until Linux has brought both CPUs up.
To accomplish this, use petalinux-boot --jtag --tcl to log all of the Tcl commands to a script file.
Then, edit the Tcl script to remove the "exit" command in order to prevent the disconnect of XSDB from the processor.