I really want to thank-you for the inciteful feedback. To give the full backstory from the programming perspective, I chose not to let players press both because it would have required another sprite and our artist was already behind with the time we could dedicate to the jam. I also assumed no one would want to do that.
To get technical: When not touching a planet, the player experiences an acceleration force relative to the distance from each plant just how gravity works, "slow down" applies a force relative to the current velocity like moving through a fluid and "increase gravity" is actually a constant force to the target planet (so technically not how gravity works, but is intuitive to players because we spend our lives at ground level). The 3 forces all being relative to different things creates a decent that is just as confusing to control if you can do both (theoretically) and perfect play would never press both.
In retrospect, that was a mistake. I didn't test that assumption and took away the freedom from the player. Will learn for next time.