One flipper slap saves are almost always going to put you in better shape than two flipper. I had to break myself out of the “one-two” slaps and saw a big difference in the type of control it results in.
Like you said, flipping both puts you at risk of “saving” it to the underside of the opposite flipper, or at best, the second flipper throwing the “saved” ball out of control, and back at risk of draining.
Practicing one-handed is a good way to help break this habit. When you’re making the slap save attempt, try flipping just a hair later than you normally would and thinking about trying to aim it at the opposite inlane. When you’re back in two-handed play, be ready with the opposite flipper to control the ball if you hit the bottom of the sling post, but don’t just immediately flip it out of habit.