Choosing between multiple signing keys

[Luis Ressel]

Hello,

today I created a new key with two separate signing keys and moved
those to my two yubikeys. But now, gpg insists on always using the
second of those subkeys for signing operations, even when the yubikey
with the first subkey is plugged in.

How can I explicitly specify which subkey should be used? I've tried
passing in the subkey ID via the '-u'-option, but this didn't work.

I'm using GnuPG 2.1.17.

Regards,
Luis Ressel