Can the second injector spots be drilled/tapped and have a second fuel (race gas) be plumbed in tandem with the vtec engage?

 

Yes.

 

Ok... Is it do'able with a P28 or do I need a better system?

 

Honda Ecu's have only 4 injector drivers. I believe there are creative ways to circumvent that limitation however a standalone EMS would be best.

 

Totally unnecessary, and extremely unreliable.
Buy a meth injection system.
Neptune, Hondata and Crome all have output options to activate solenoids that can be used for meth injection, staged boost, or nitrous.

If you're boosted, many meth injection systems ramp up volume based on pressure, that would be my recommendation. Xenocron has a few:
Xenocron Tuning: Snow Performance Water Methanol Injection Kits for Honda, Acura, Subaru, WRX, EVO, Nissan, Mitsubishi, BMW, Mercedes, Diesel

If you're naturally aspirated, unless you're above 12:1 compression You'll see virtually zero benefit from using methanol OR race gas, but FWIW the single-stage systems are about $100 cheaper.