Use Adobes Flash Media Live Encoder as encoder instead and you should be able to use just one computer. Im on mac and I use cam twist, fmle and ATEM tv studio and my cpu is at 50-60 % even when I do "heavy" stuff. Use your other mac pro for extra cam twist stuff if you need and hook it up to the the switcher if you have a free hdmi input. Cam twist is indeed heavy on your cpu but on a mac pro it should not be 100%.
On Livestream I don´t think you need cam twist they have their own broadcasting software if I remember right and that software is pretty cpu intensive too. So if you use cam twist in addition with the livestream broadcastingsoftware your cpu will have to work really hard. Also on the free accounts on lifestream you cannot cast over 500 Kbps which is way to low to get a good enough quality.
This is just a test broadcast I did. If you let it play for a couple of seconds in fullscreen its going increase the quality. I made a misstake with the sound though so mute that.
Check out the very last few minutes of this broadcast too it turned out very cool even if it was very dark outside.
These broadcasts where done with ordinary cheap consumer video cams one panasonic and one Sony. I have the Intensity Pro capture card but the mini recorder should be just as good maybe even better.
When I watch on my cheap 50 inch flatscreen in fullscreen its pretty ok actually so you should be able to get the same quality,
Good luck