Sorry if it wasn't too clear, I was trying to keep it simple and assume Person B sells all BTC to USD and vise versa when he buys. So he buys 1 BTC @ $2k the price doubles and ends up at $4k (sells the lot). He buys $4k worth of BTC when it hits $2.5k and ends up with 1.6 BTC. Sells everything again at $4.5k and ends up with $7.2k. Net profit of $5.2k, or worth a total of 1.6 BTC with his initial investment included in that.