Here is an example of multi-tasking / multi-threading:

# declare functions
function a() {
 echo "a";
}
function b() {
 echo "b";
}

# register tick functions
register_tick_function ("a");
register_tick_function ("b");

# make the tick functions run
declare (ticks=1);

# that's all there is to it.

Notes:
This will make functions a and b run once each at the same time.

If you try:

declare (ticks=1) {
 1;
}

They will run twice each. That is because it seems to be an undocumented fact that there is always an extra tick.

Therefore:

declare (ticks=2) {
 1;
}

Will cause them to run once.