How can I manage multiple fulfilment steps for a single order?

[Ttimbo123]

Hi Shopify community! - I'm trying to wrap my head around a logistical issue and would love your help.

I'm a single product store (for now) and there are 2 steps required for me to fulfil an order.

Step 1: I post the customer an item which requires a customer action
Step 2: After the action is complete, I post a secondary item and fulfil the order

As such here are some questions:
1. How do I partially fulfil the order and send a postage tracking for the 1st item?
2. How can I then notify the customer that we've received their action and are preparing the secondary item
2. How can I then complete the order and send through a secondary tracking number for the 2nd item?

Each email needs to different triggers and templates,

What's the best way to achieve this?

Cheers,
Tim