Well, maybe too late on this, but I make lager starters a week or more in advance. It takes that long to step them up and properly crash them. Plus I like to ferment my lager starters on the cold side. If I make it closer to brew day, I'll usually just pitch the whole thing (2L/5gal.) and not worry about crashing.
If you added the other half gallon last night, I'd crash it ASAP and as cold as you can without freezing (maybe 29°F). It won't cause any problems crashing before it's finished. If you do crash it, don't forget you have to warm it up closer to pitching temps before pitching... so I would decant first, then warm up. That way the yeast don't start fermenting again and become suspended before you decant.
It would have been good to add yeast nutrients to the starter, but a PM really shouldn't need it because there are plenty of nutrients in the grain. Can't hurt though.