Make sure the weapon ("Default Primary" in properties) for the first entity is different from the second entity (e.g. 12mm Pistol for guard1 and Assault Rifle for guard2)
Multi select both, go to properties, hit OK
Both now have the same weapon selected (12mm Pistol in the example)
For entities where the weapon is valid for both, going into properties for either will now display the same weapon.
For entities where the weapon is not valid (per "Allowed Weapons" in entity.tbl), going into properties will display "none" for "Default Primary", however they will still use that weapon if you save and test the map in-game. For example, see the screenshot below, guard2 (left) cannot normally use a pistol, but I edited the properties of both him and guard1 (right) who does use a pistol. This overwrote the selected weapon for guard2 (Assault Rifle) with a pistol, even though guard2 normally can't use a pistol (and lacks animations for it).
To reproduce:
For entities where the weapon is valid for both, going into properties for either will now display the same weapon.
For entities where the weapon is not valid (per "Allowed Weapons" in entity.tbl), going into properties will display "none" for "Default Primary", however they will still use that weapon if you save and test the map in-game. For example, see the screenshot below, guard2 (left) cannot normally use a pistol, but I edited the properties of both him and guard1 (right) who does use a pistol. This overwrote the selected weapon for guard2 (Assault Rifle) with a pistol, even though guard2 normally can't use a pistol (and lacks animations for it).