Ok, so as per my understanding, you are splitting a single array into two and then trying to encode them parallelly and then concating it back to one array.
If this is the case, I would suggest to split array in chunks, create a workflow to encode then use parallel activity inside it invoke the workflow to encode and check Isolated property. This will ensure both are working parallelly in separate UiPath executor processes.
Use Out argument from workflow to encode. Concat the output of each invoke workflow to encode into one array outside of Parallel activity.
Connect personally if we are at right direction and stuck somewhere!