My understanding is to basically start with a full load. By this I mean the appropriate amount of fish - not overloading your tank. But, if you have put in the right amount of Bio-Spira in your tank, and it has been handled properly (meaning the bacteria are still alive) you will need the amount of ammonia produced by a stocked tank to let the bacteria thrive.
That having been said, if you have already added bio-spira with only 2 fish and it has been over 24 hours, then you may want to allow the cycle to progress at a slower rate and add more fish in a couple of weeks to slowly increase the ammonia to keep up with bacteria growth. If it has been over 24 hours with such a small number of fish, you may have already lost a significant amount of the bacteria.
Your other option would be to add more bio-spira along with the additional fish. The good news here is that you can't really overdose with bio-spira.
Hope this helps.