Can I register channels from different sub-keys with a given device?

Yes, but note that a channel registered to a device is exclusive to a sub-key.

To register device foo on channel my_channel for sub-keys sub1 and sub2, you need to make two API requests:

curl apns.pubnub.com/v1/push/sub-key/[sub1]/devices/foo?add=my_channel

curl apns.pubnub.com/v1/push/sub-key/[sub2]/devices/foo?add=my_channel