Yes, one right after the other, just not together. I dose Alk first as I can see it as it disperses. As soon as I can no longer see it I add the Ca. If possible add it to a sump or fuge, if not add slowly to a high flow area.
I'd avoid "Marine Buffer" for use as an alkalinity supplement in reef tanks. Seachem Reef Buffer and Reef Builder are OK, essentially being baking soda and baked baking soda. Marine Buffer has borate in it, which is not desirable as a reef additive for alkalinity.