Even if my client just watches.Seems a bit harsh.
It's not harsh at all. You are facilitating his enjoyment of watching you with this other guy and also allowing him a safe space to engage with the guy too if he so wishes. Also bear in mind that if this other guy is a young male escort then chances are he will maybe be going at it a damn sight harder than the client may have done, feeling he has to put on a good performance so to speak and quite likely for his own pleasure.
Even if the client doesn't have sex with you he is still going to get off on the voyeurism.
Otherwise it's almost the same as some random guy saying " Well he's already paying you to have sex with him so can I just come in and watch and wank for free?" Can't imagine many of us would allow that.
It's over and above what he would be getting in a normal booking so why should he be paying your normal flat rate?