Does Using Multiple Hardware Wallets Actually Make Self Custody Safer?
It depends on how they're used. Trezors are fully open source and the code is arguably the most used among hardware wallets. Other devs use it as part of their own work as well.
I think the average user would be better off with 2 Trezors, so they can wipe one out to test their ability to recover their wallet from scratch without risking losing access to the wallet, since the wallet is also on their other Trezor.
I understand that's not what you mean by "multiple hardware wallets" though.
For anyone who is securing more than a few thousand bucks worth of Bitcoin, I strongly believe the answer isn't more hardware wallets. No. The answer is a hardware wallet that is stateless, which means it doesn't save your seed or wallet on the device. SeedSigner, ShieldSigner, Krux and Kern are examples of stateless hardware wallets.
For serious self custody, I believe a wallet should be airgapped, stateless, and using a seed generated by a person, not a device.
Airgapped... so online thieves can't reach it.
Stateless... meaning the seed and the wallet get wiped when the device shuts down. If a thief steals it, there's nothing on it.
Human generated seed... roll dice 100 times. Or, print the BIP39 word list in columns so you can fit as many words on a sheet of paper as possible, along easily cuttable lines. Cut the paper into strips so each word is on a strip. Put the words in a large mixing bowl. Shuffle them. Pick a word. Write it down. Put the word back in the bowl and shuffle again. Pick another. Do this 23 times. It's perfectly safe to let a hardware wallet calculate the checksum word since you know the other 23 words are truly random.
Also, pick 10 more words. There's your passphrase, and it's uncrackable.
Currently, I believe the best hardware wallet at any price is ShieldSigner. It's SeedSigner with extra security features such as encrypted seed QR and passphrase QR. Passphrase QR makes loading a very strong passphrase easy. Krux is excellent too, and the hardware to run it is cheap. Kern is a newcomer to this space, but I think it will be one of the best options a year from now as the project matures.
This kind of setup isn't right for newcomers. But once somebody has experience with wallets and understands that the seed is the wallet, not the device, then working this way makes sense and offers incredible security.
The problem, of course, is that most people think the device is the wallet, and people have a weird need to put their faith in a brand.