Most yeast manufacturers say that a 5 gram packet of dry yeast is for 1 to 5 gallons of must.
I once used a packet of yeast divided into forths for 4 different gallon jug batches, and got perfect results.
The whole 5 gram packet is not too much for one gallon!
But, once you open the packet and only use part of it, you had better use the balance pretty quickly, or it will run the risk of contamination even in the fridge.
If I had more batches planned in a day or so, I'd probably risk it. Other wise, you will most likely lose what you don't use and then contaminate and lose some expensive fruit or juice when you do get around to using it.