pwm controlled by relay

[usbcd36]

You could use static variables to achieve these means.

static unsigned char Tank_Drive = 1; //default is tank drive

if (p1_sw_trig) Tank_Drive = 1; // enable tank drive
else if (p1_sw_aux1) Tank_Drive = 0; // enable other drive system

if(Tank_Drive)
{
     // code for tank drive here
}
else
{
     // code for other drive system here
}

What this will do is switch to tank drive when you press the trigger and switch to another drive system when you press the aux button. Not when you hold them down, but when you press them.