Apache supports advanced load balancing.
I dont know if it works (and anybody tested and if worked please comment and say it works) cause I am now reading about it, but in
at the slide 22 says that you could use:
BalancerMember http://www1.example.com:80/ loadfactor=1
BalancerMember http://www3.example.com:80/ loadfactor=1
BalancerMember http://www2.example.com:80/ loadfactor=4 status=+h
something like this and you notice there is
loadfactor I think it is what you are looking for.
There is another option, at slide 31 it says there is
smax parameters which are:
• Initial number of connections 
• Hard maximum number of connections [1|TPC]
• soft max - keep this number available [max]
And again if someone (or you yourself) tested it and worked properly please comment out and tell me to change this answer to tell others that its working.