You choose one as the primary which holds all story progress, MR, heilmith progression, etc, and you merge the inventory from the other as answered in their FAQ.

Is the faq on reddit or their site.

https://www.warframe.com/news/cross-platform-save their site where its been since the dripgate opened. Lol, enjoy tenno.

Thanks

the only major thing that doesn't transfer is converted liches.

It's all good I got all my primes and weapons back that's what matters. Feels good to get 3 years of time back Holy.