Adding more yeast within the first 24 hours

Does it help?
I made a robust porter yesterday, 10 gallons 1.077 OG. using WLP002 I made 3 litters starter. The starter fermented for 2 days and sat in the fridge the third. Decant the starter down to less than 1 liter and I visually divided it between my two 5.5 gallons carboys. Now I feel that I under pitched both carboys, but one more that the other. Would it help I go to the LHBS right now and buy a tube of WLP 002 and pitched it in? One carboy has started fermenting the other not yet.

Thanks

It won't hurt to throw another vial in each. I you just want to dry it out you can throw in the Dry English or British after one more day - it won't change the flavor but will help dry it out. You could probably even save $$ and use a packet of the S-04 instead of more liquid but you'll want to rehydrate the dry stuff so it doesn't get stuck in the krausen. If you're planning on rinsing/reusing the yeast you'll have a blended yeast, though.

But do you think I need to? What would go wrong if I under pitched in a porter?

You don't need it and it is too late anyway. It would just be a waste of money.

You would only add more/new yeast if the original yeast was dead.