Mix the mascarpone with 1 tablespoon of the capers, 1 tablespoon of the chives, and the lemon juice. Season with salt and pepper.
Lay the salmon slices out on a cutting board so that they are all arranged with a short side facing you, with plenty of space above and below each slice. (If necessary, fill and roll the slices in batches).
Put a spoonful of the mascarpone mixture on the narrow end of 1 slice. Lay 2 arugula leaves, fanned slightly apart, across the mascarpone, so that the tips of the leaves will project several inches from one end of the roll, and roll up the salmon. Stand the roll upright, the arugula leaves pointing upward, on a platter large enough to hold all 24 rolls. Repeat the process with the remaining slices. Cover and refrigerate until serving.
Remove from the refrigerator, sprinkle with the remaining 1 tablespoon each capers and chives, and serve.