I don't believe you can edit existing monsters yet without editing the core game files, which would mean it's not possible to upload the mod to Steam and it wouldn't be possible to run it alongside other mods that edit those core files. You could make your own monDB.xml with monsters and add <drop name="a specific item" percent="a percentage"/> or <drop type="artifact" percent="a percentage"/> inside a <monster> tag to make that monster drop items.
I tried. But it is giving only one drop. I've put two drops at 100 percent. So it should give both. But it is giving only one.
You can try a workaround which is used in the OneMoreNameless' ASCII mod, used at least for his miniboss Your 1000th Dog: in the monster tag, alongside sprites and secondarybuffs the monster has, he put on the <onDeath spell="Loot Sweet Loot" percent="100"/> line, which recalls Code: <spell type="targetfloor" name="Loot Sweet Loot"><effect type="triggerfromlist"><option name="Loot Bolts 1"/></effect><option name="Loot Bolts 2"/><option name="Loot Bolts 3"/><option name="Loot Bolts 4"/><option name="Loot Bolts 5"/> And it works 100% of the times.