Well, disregarding what Fermentis says, lagers are best fermented cold (at or below 55°F). Most are good around 50°F, but some can go into the 40s without problems.
The ideal way to have done it would have been to pitch 2 packets (hydrated) at about 45°F and let it warm up to 50°F. But the question isn't what should you have done, it's what to do now.
I wouldn't pitch another packet. You're already fermenting and it won't do any good. I've heard lots of stories about "fruity" lagers when using many dry strains and fermenting them warm like yours is, so you might end up with a fruity lager. Most of these fruity esters are produced early in fermentation, so there may be no way to prevent it now. Plus if you crash it down to 50°F, you might shock the yeast and get a lower attenuation than you want.
I'd lower it by 5°F today, 5 tomorrow, and 5 the next day. That way you won't shock the yeast and you might reduce a little ester production. Then when it's about 3/4 done fermenting, warm it back up (you can warm up quickly) to the 60's for a couple days so the yeast can clean up diacetyl and other compounds. (A low yeast pitch rate and initially warm fermentation temps can also contribute to diacetyl production.) Then slowly lower to near lagering temps (5°F per day), rack to secondary, and lager.
If it's a medium-gravity to higher gravity lager (say, anything over 1.045), I'd lager it for like 6 weeks to try to clean it up a bit. In any case, it should still be a drinkable beer and may turn out better than predicted.