According to Halachipdeia (source) you're not supposed to walk 4 amot in front of a person praying the amidah.
I think you also have certain restrictions regarding which parts of kedusha could be answered prior fully completing your Amidah (taking three steps back and then three steps forward), unless perhaps there's an earlier period where this restriction is removed (perhaps the second yihyu leratzon).
What should you do if the chazzan began the repetition but the person behind you is still reciting the amidah? Do you stay where you are and say the kedushah with whatever restrictions are imposed on you (such as, I've heard, not saying the first sentence of kedusha - nekadesh/nakdishach)? Or does saying the full kedusha take precedence over walking within the 4 amot of a person praying behind you?