Well, "targets target" is a quite simple option in Isboxer. You didnt post your setup so I cant help you finding the mistake so I just try to explain how it works for me.

While you have to use a keymap there are two ways of doing it. For me it works to just do a one (1) step keymap. Step one (1) send the key you need to press to target the group member you want to assist to your slave window (for me it simple is "F2" to target my group leader) and in the same step (still step 1 !) send the key to get targets target (for me its "#"). I had trouble using the premate "ctrl+t" so I switched to "#".

If your setup uses 2 steps (not needed but you can do that) its a must you set your keymap to execute a step when hotkey is "pressed or released". Than just do it like written before.

While you didnt post your setup I would guess you did it while using a 2 steps keymap and didnt set it to "pressed or release", right ?

Hope I could help.