No, you can not use 2 computers on 1 FireWire input. The second FireWire port is meant for either cascading AI consoles or adding an external harddisc. PreSonus Knowledge Base says:
"There are two standard 9-pin FireWire s800 ports on the back of the StudioLive. Either port can be used to connect the StudioLive to a FireWire s800 port on your computer. Use the second FireWire s800 port to connect additional s800 devices (such as external hard drives) to your computer." (https://support.presonus.com/hc/en-us/articles/210047673-Option-Card-Overview-and-Connections)
This sentence can also be found in the RML owners manual, section 3.10 FireWire Audio Setup / To set up the FireWire connection.
You need to run Capture and MultiRack on one computer, that's the possible only solution. Waves also has a (free) multirack recorder...
