Porting the twohanded component.

code/modules/mob/mob.dm

@@ -745,7 +745,11 @@ GLOBAL_VAR_INIT(exploit_warn_spam_prevention, 0)
 	return FALSE
 
 /mob/proc/swap_hand()
-	return
+	var/obj/item/held_item = get_active_held_item()
+	if(SEND_SIGNAL(src, COMSIG_MOB_SWAP_HANDS, held_item) & COMPONENT_BLOCK_SWAP)
+		to_chat(src, "<span class='warning'>Your other hand is too busy holding [held_item].</span>")
+		return FALSE
+	return TRUE
 
 /mob/proc/activate_hand(selhand)
 	return