[mikets]

You should have a scaleFactor variable and have the buttons modify it.

bool j1Btn1 = J_stick1->GetRawButton(1);
bool j2Btn1 = J_stick2->GetRawButton(1);
float scaleFactor = 1.0;
if (j1Btn1 && j2Btn1)
{
    scaleFactor = 0.25;
}
else if (j1Btn1)
{
    scaleFactor = 0.5;
}
else if (j2Btn1)
{
    scaleFactor = 0.75;
}
 
R_myRobot.TankDrive(J_stick1->GetY()*scaleFactor, J_stick2->GetY()*scaleFactor);