Tested using md(4) memory disks to confirm multiple providers can be attached if they use the same passphrase and keyfiles, but each provider fails to attach if the passphrase / keyfile combination fails to decrypt its master key.
```
mdconfig -s 1m -u 0
mdconfig -s 1m -u 1
mdconfig -s 1m -u 2
mdconfig -s 1m -u 3
geli init md0 # set password to "test"
geli init md1 # set password to "test"
geli init md2 # set password to "test123"
geli init md3 # set password to "test"
geli attach -v md0 md1
geli status
geli detach md0 md1
geli attach -v md0 md1 md2 md3
geli status
geli detach md0 md1 md3
```
Expected output:
```
Attached to md0.
Attached to md1.
Done.
Name Status Components
md0.eli ACTIVE md0
md1.eli ACTIVE md1
Attached to md0.
Attached to md1.
geli: Wrong key for md2.
Attached to md3.
geli: At least one provider failed to attach.
Name Status Components
md0.eli ACTIVE md0
md1.eli ACTIVE md1
md3.eli ACTIVE md3
```