Nope, you don't need two subscriptions
We've designed CrewBuddy to automatically honor the subscription you have on your iPhone. So, as long as you have a valid subscription on your iPhone, then CrewBuddy on your iPad will automatically register and honor it.
Just make sure that you connect your iPad to a real WiFi (not aircraft WiFi), for it to automatically register your subscription. More information on why: https://wapitisoftware.freshdesk.com/support/solutions/articles/13000009065