In concept this sounds nice, in practice I think this wouldn't be practical.  The Advent already have exceptional shield regeneration capacity coming from the Progenitor, and the repair platform is really their only way to heal large numbers of hull points in any reasonable amount of time (the Subjugator comes very late in the tech tree and isn't very good even then).  That platform is pretty antimatter hungry, so it really doesn't have the reserves to juggle two abilities.  The weaker ability will get turned off, and since hull points are way more valuable than shield points to begin with and Advent has other ways to regenerate shield, I know which mode will be used every time.

So, in my opinion, this is an ability that looks good on paper but in practice it's just not useful compared to your other options.

Shields actually regenerate pretty quickly between battles.  If you're anticipating another major battle within 2 minutes or so, I certainly would not be blowing repair platform antimatter lightly, and would conserve it only for hull points.

We're having a problem with the Vasari hanger right now on similar grounds.  It's not that producing strike craft is a bad investment, it's that phasic traps put that antimatter to much better use.  The weaker ability gets ignored because this is a structure with very tight antimatter capabilities.  The same thing will happen to the Advent repair platform if you add a secondary ability, and as a rule shield is less valuable than hull for single-target abilities (they're about equal for multi-target abilities).