This should do it.
void doublestep (int steps1, int steps2, int dir1, int dir2, int style1, int style2) {
while (steps1 || steps2) {
if (steps1--) {
motor1.step(1, dir1, style1);
}
if (steps2--) {
motor2.step(1, dir2, style2);
}
}
}