You also might want to check out the
Reef Chemistry Forum on Reef Central.
Randy Holmes-Farley has written some decent articles and has answered a lot of questions like what you've posed.
What I know from experience so far is no, you can't dose them both at once. You can't use the bicarbonate in your limewater mix.
Also, is your Ph usually lower? Higher than 8.5 and I'm usually concerned myself. The kalkwasser dosed too quickly can cause a rise in Ph. You'll also want to make sure your
kalk mix is fully saturated, 800-900ppm Ca and 40 meq/l Alkalinity. I find that takes 2 tsp per gallon of RO water.
In my tank since I run my refugium lights 24/7 I can't dose kalk 24/7; it raises my Ph too quickly. I have to suppliment with .8 tsp of bicarbonate (superbuffer dkh) per day to maintain my Alk. I buy the 1lb containers of dry superbuffer dkh and mix up my own 1 liter container of it. The dry buffer should be premixed in water, you can drip it, or add it straight to the tank as long as you aren't dosing large amounts.
My Ph stays 8.2-8.4 constantly and my Alkalinity stays up at 3.2 meq/l (8.9 dkh) this way.
Wes
