Just add more Arc processes to the balancer section. Apache listens on port 80 and each Arc process listens on a different port 8080, 8081, etc.
The mod_proxy_balancer module is what many folks use to put Apache in front of a bunch of Mongrel processes when using Rails, the same approach works for Arc, but there are cluster implications (see the parent of the link above http://arclanguage.com/item?id=3486) because Arc doesn't use a 'shared nothing' approach.
In particular, I think you'd at least want to use sticky sessions so the same browser is always routed to the same Arc process where the correct continuation(s) would reside.
I got it to work behind apache with mod_proxy. One gotcha I never got around to fixing was getting the arc app to pay attention to the X-Forwarded-For header instead of just using 127.0.0.1 as the remote ip address.