When creating a Linked Wearable with the new UI, it prompts for a Smart Contract Address and you can select which network. You cannot select multiple Smart Contract addies.

Why would anyone want a wearable linked to 2 different collections?

It would be good for ecosystems that have smart contracts that create alternative collections; ie Doodles & Space Doodles. If you soft stake a Doodle and receive a Space Doodle, then your Doodle is no longer in your wallet although the 2 NFTs are linked and selling one sells the other.

You can already link a single wearable to multiple NFT IDs, but they must be within the same ETH Collection.