unit Royal guards. We have 3 guards with the normal stick and one guard with the electrostaff.

Does this mean that if the RG are in melee with two enemy units, the RG can attack both of them?

one guard Attacks one unit with the electrostaff and other guards attack the other unit with normal sticks.

Normally units cannot split Attacks because model with the same weapon MUST attack the same target; and normally units have only one melee weapon

There is no special prohibition against attacking multiple targets in melee. You can split attacks from different weapons as normal.

33 minutes ago, toffolone said:

unit Royal guards. We have 3 guards with the normal stick and one guard with the electrostaff.

Does this mean that if the RG are in melee with two enemy units, the RG can attack both of them?

one guard Attacks one unit with the electrostaff and other guards attack the other unit with normal sticks.

Normally units cannot split Attacks because model with the same weapon MUST attack the same target; and normally units have only one melee weapon

As @arnoldrew said, yes you can. It is even mentioned in the RRG under Melee Weapon on page 39:

"If a unit has multiple minis each using a melee weapon, each melee weapon with the same name must be added to the same attack pool. Melee weapons with different names may be added to different attack pools ."