Your lightning wallet will use addresses from a different HD derivation path, so chances are if you input the same seed phrase, you won't see your bitcoin-wallet funds available in your lightning-wallet anyways.
You only have to store one seed phrase, perhaps simplifying offline storage considerations.
Lightning wallets are hot wallets, so your private keys will be stored on an online device. Any security breach could compromise not only your lightning wallet, but also the rest of your funds.
Personally, I would not use the same seed phrase for both. I think it is MUCH smarter to compartmentalize the security of each wallet, especially considering the nature of lightning wallets.