As far as Ginobili starting and running the second unit, he can do both. That's exactly what he did last season. As far as a general outline for a playoff rotation, just sub him out at the 5 minute mark, bring him back for Parker at the 1 minute mark, bring Parker back at the 8 minute mark, sub Ginobili back out, then bring him back at the 4-5 minute mark. That's roughly 19 and 16-17 minutes, respectively and 11-12 together, per half.
The best players need to play together for significant minutes. He already refuses to pair his fourth best player with his three best (I mean collectively, not individually) and they never play what's statistically their best five. They can't continue to compound that by not playing their three best together that much.