Sure, but only clients that implement this NIP will tag both. Majority of the clients will just do the basic one and tag the subkey. Which now turns into a mess because some clients work others don't. 

+ For DMs, the encryption happens to each key. You can't just tag both. You have to create a NIP-17 group with the 3 of you (2 of your keys + 1 for your friend) so that you can reply to the other two at all times.