Infinity with TimeLeap

I doubt it. I bet it works just like any summon. If you try summoning it again, the first will disappear and a second will appear.

that isn't how "any summon" works anymore. a la reaper some summons are only a matter of how many you can pop out there.

I'm not familiar with the theif skill in question however so if it does actually put an effect on your character then i would assume no they dont stack. If it works more like an attacking summon then i really can't say.