Saving leftover liquid yeast?

Hi guys. I'm relatively new to brewing and still trying to avoid silly mistakes.

My friend gave me a smack-pack of wyeast belgian stout yeast. I am making some stout, so I thought I'd use it. The trouble, I just realised, is that the smack-pack is good for 5 gallons and I am only making two gallons in my little fermenter.

If I pitch half of the yeast, is there a way to save the rest until I have time to brew again? I have two fermenters. I can't do them both at once because I used all my pots. I'm brewing with all grain in an apartment. 2 gallons at a time is all I can do.

P.S. I already "smacked" it! Time is of the essence!

What's the OG on the stout and what is the date on the smack pack. depending on these you might need the entire pack.

woah, I don't know. I haven't learned to check out the gravity of wort yet. I'm not sure I will, either. I'm too lazy. I hope no-one thinks I'm being unfaithful to beer, but I am steering clear of the science stuff and just keeping notes of what I did, and whether the beer was any good or not. So far, all the beer has been good, but I don't know how much alcohol it has or anything like that.

I just pitched a package of dried yeast to be on the safe side. The smack pack wasn't showing any signs of life five hours after I smacked it. If it inflates sometime in the next few days I will make another batch of beer on Sunday. Maybe I will pitch the whole thing then. Is there a negative consequence from pitching twice as much yeast as necessary? I mean, it multiplies in the beer anyway right?

I figured after spending five hours making the wort, I'm not into experimenting with funky yeast. But I would like to not waste it.