That's what I've been doing up until now...having two keyboards on stage, one of which was capable of producing its own output (e.g. Yamaha CP33) as a backup to Mainstage.
For this band I'm keen to stick to one keyboard only...the portability of the SL73 is great and I'd like to stick to using it rather than the CP33.
So I guess I'm looking for the most compact backup option...I will have a small table beside me with laptop, mixer, breath controller etc, so it would be trivial to add a small module like the XV2020 as a backup sound source, taking midi from the SL73.
Some great feedback here guys...I'll let you know what I decide to do.
I was thinking the MX49 would be good (sitting offstage/nearby/unseen/etc.), just in case the SL73 goes down. You could have the MX49 power/audio cables always ready to go on the stand, and just switch out boards quickly. Just as a last resort (backup of backups) kind of thing.