Shoving two pellets up the barrel is just going to give you dreadful accuracy.
Accuracy is everything with airguns as even a very powerful springer is far less powerful than the weakest firearm. Get the most accurate pellet you can find, usually JSB or one of the branded versions of such.
Airgun pellets are soft and hollow and will deform on impact, no need for hollowpoints or spikes or any kind of weirdness, just use round-heads or flat-heads at the shorter distances.
If you want to use expanding bullets, get a .22 rimfire bolt gun and knock yourself out.