You say the B and the G strings, but in most slap/pop ( at least the most straightforward or easiest anyway)the lower note is hit by the thumb and then pulled with the index finger an octave above. So slap on A string pop on G, slap on E string pop on D string. Obviously you can slap on the B, but it can be difficult to get this right and it is much easier to go for the octave thing, with the pop on the G string. The B can be too thick or too high to slap easily or rather conistently. It might better to start trying this on the A and G strings and then progress when you are confident.
Of course - all this might be at a different level to where you are now and this is probably why others haven't replied. Without seeing you play, it is very difficult (impossible!) to comment and suggest improvements. This is why it is best to have a teacher really - a teacher can *show* you improvements in a few minutes that it would take pages on here to write - and they might not be appropriate anyway!
