How mobile was the Macedonian phalanx?

[Ludens]

I recall that sarissa can be separated in 2 parts

maybe they are sprinting using only the one half of sarrissa as normal spears

I doubt the enemy would be so obliging as to wait while the pikemen were reassembling their sarrisa after the charge. So any charging pikeman would have to fight with a normal spear during combat. In which case, why didn't they simply front the phalanx with hoplites, who have a spear and a proper shield as well?