I did a lot of digging around 3 or 4 months ago looking for the right doser to automate this. Honestly, I think there are a number of good dosers out there. I finally chose the Aquamedic Dual reef doser. It's easy to use, very quiet, each pump can be controlled independently and the on/off times can be set in seconds, minutes, hours, so lots of flexibility:
http://www.premiumaquatics.com/Merc...OD&Product_Code=AM-70105&Category_Code=Dosers
Others will recommend the litermeter. Probably a very good pump, but expensive. And you still need to buy a second pump module to do what we want here.
The Sentry systems dosers are also recommended a lot. This one can do both solutions with a single pump. In the off-chance you want to dose different amounts of the 2 parts (you shouldnt really do this anyways), this would lead to less flexibility. The only legitimate reason I can think of for doing this are if you make up your own 2 part solutions, you may find that one of them is slightly off in concentration in one way or another.
I had 2 issues with the Aquamedic. First, I got one sent to me with the wrong pump installed:
http://reefcentral.com/forums/showthread.php?s=&threadid=739362
I got that tyaken care of. Second, I would suggest to everyone using them that they do a couple simple tests up front to check the dosing rates on each pump. Mine dose slightly differently from one another - one doses 25ml/minute, the other 30-32 ml/minute. If you know the rates, you can adjust the on/off times to correct for that.
Finally, if you decide to use Randys 2-part, keep in mind it is approx half the conc as normal B-Ionic, so doubling your dosing volume is a good starting point. I have a very high Ca demand in my 75G, and I was adding 60+ mls of B-Ionic daily and still not maintaining levels when I finally decided to do something. Now I am doing about 150ml/day of Randy's and I'm almost where I need to be. Maybe 160-175 should do it, I hope. Plus, I get to add it continually throughout the day - I set my pump to come on every hour and add 6-7 mls. Way better than dosing by hand.
Better than a Ca Reactor? - who knows. I'm sure there are pros and cons. But I am going to give this a good chance to work before making the change.