For a few months I've been using SSD to layer a snare and kick drum on top of Superior, but I've been doing it in a way that's a bit of a faff. I would duplicate the Superior MIDI track and paste it onto the SSD MIDI track. It works fine once you've finished the song, but it's a bit shit to do whilst you're programming out the drums.
I just did a little searching and found out how to send one track to two VST instruments. Stupefyingly easy, if you already know how. Not so obvious, maybe (wasn't to me), if you don't. Thought others might find this useful.
So, you have the MIDI track (in my case, the Superior one) that you want to program all your hits out on. Click the little 'e' icon for that track to open its properties.
In the MIDI Channel Settings window that appears, click the power icon on one of the MIDI send slots (there's four, choose which one you want to use - I use slot one because I'm not a whackjob), then click on the Out dropdown and choose the second VST you want this MIDI track to trigger.
Obviously, you can send one MIDI track to up to four VSTs with this method. But I only need two for now.