Normally, Multishot and Piercing cannot be obtained on the same crossbow. This method is very difficult to obtain but allows the enchantments to be obtained as well as double enchantments.
The basics for obtaining this item would be to kill a very high number of pillagers. The most recommended way to do this would be to build one of the following raid farms in your world:
In minecraft a rare drop from the pillager is an enchanted crossbow with incompatible enchantments.[1] Here are the probabilities for the following combinations of enchantments:
