Generally yeasts are pretty sensitive to high levels of oxygen and high temperatures (Obvious when you fail to cool your wart before adding yeast - one of the many benefits to owning a wart chiller). Early in my brewing I attempted to split my yeast twice, once successfully to both primaries and once where it only worked in the first primary. Although, my sample is small, in the fridge it lasted a month.
I've heard that you can store it in the freezer for up to 6 months (I guess this happens in baking), but I can not speak on experience with this in brewing. Nor do I recommend do this.
Instead I would suggest taking some brew off the top of your previous as it should be only be a few generations old. Look into "repitching techniques" http://byo.com/grains/item/739-harvesting-yeast-techniques. It's a fairly common technique.