Only one arcanist needs to cast arcane circle and that one that casts is the one that the point spread check is made on. Also, going to prism of light can be worth it in that the arcane circle in the rest stop there has a bonus circle that can make a focus of SIX! If you are going to be chaining a lot of peerless or doom or whatever, It really is worth the 10k to get in the prism for that bonus for Word of Death alone.