"Really." Snape replied to Sirius, as coolly as ever. He drew his wand - and if you could watch him do it, you knew he was doing it with unnatural slowness, purely to make a point.
"Do you want me to see if you can learn to resist it?" He kept his questioning stare on Sirius for a long moment, before flicking it toward James. Because, as annoyed as Snape might be by the vulnerability of the arrangement, he recognised that the two of them were now a package deal.
Training one was training both. Protecting one was protecting both.