semi-automated water changes

[Mary Lindstrom <lindstro at biostat_wisc.edu>]

I do hope this topic hasn't been done to death.  I did search the
archives and came up with the following approach.

My situation is a 30 gallon tank. I would like to do 10% water changes
with a minimum of fuss.  We have very hard water (Madison, WI) so I
buy RO water for 2/3 of the new water.  The local tap water has a
small amount of chlorine, no cloramine.

My proposed approach involves having a 4 or 5 gallon overflow
container under my tank which I would empty manually.  I would also
have two additional containers on the floor: one with RO water and one
with aged tap water.

The goal is to 

   move 3 gallons out into the overflow
   move 2 gallons of RO and 1 gallon of aged 
       tap water in from separate containers on the floor.
       with no lifting and minimum intervention (low cost is a plus).
 
I can easily set up an exit siphon to empty the tank to a preset
level.  My question is, what is the best way to get the correct
mixture of new water into the tank from the two sources. Is there a
way to pump both sources of water at once (1/3 coming from the tap
container and 2/3 from the RO container)?  I would also like to be
able to just start the pump and have it shut off manually using (I
guess) a float valve or timer.

I would rather avoid pre-mixing the 2 water sources by hand since that
involves more lifting and more buckets. 

TIA,  - Mary