In a skillet, cook the shallots in 8 tbsp of olive oil over medium-high heat, stirring consistently until the shallots are golden - about 4 to 5 minutes. Add the vinegar and grapefruit juice and cook over moderate heat until liquid is syrupy and reduced - about 5 minutes. Add cream, salt, pepper and boil for 1 minute (being careful you don't scald the cream). Reduce heat to low and add a few tablespoons of butter, whisking constantly. Add remaining butter pieces, a few at a time, whisking constantly and adding new pieces before the previous ones have completely liquefied. Lift the pan off the heat occasionally to cool down the mixture. Remove from heat and set aside.
Pull off and discard any muscle tabs still attached to the scallops. Pat the scallops dry with paper towels and season them with salt and pepper.
In a large, heavy-duty nonstick skillet, heat the remaining 2 tbsp olive oil over medium-high heat. Add the scallops (in batches if necessary to keep them from crowding), cook until they are golden brown on both sides and almost firm to the touch, 2 to 4 minutes per side. If you have an oil splatter screen, it would be a good idea to use it while cooking the scallops - but it's not nessicary.
Add the shallot sauce to the scallops, and cook briefly, shaking the pan, until the sauce is reheated. Serve immediately and sprinkle with chives if using.