With an initial gravity of 1.042, you are probably ok. I typically pitch 3 sachets of 34/70 or S-189 in my 10 gallon batches, but lately, I have been doing 5 gallon batches of 1.040 or lower lagers. They turn out fine with one sachet of dry lager yeast. A little bit slow to start, maybe, but I have done 10 gallons split into 2 5 gallon fermenters, with one being a re-pitch of freshly harvested slurry, and both finished about the same time - despite the lag on the dry pitch.
Just my 2 Lincolns here...YMMV, of course.