I know the common wisdom is you shouldn't do this, but I am brewing two mid-to-high gravity beers on Saturday (1.065 and 1.072), and only have one packet. I am now chilling a 2L starter that I was going to stir plate, decant, and split between the two batches. Is this going overboard, or should I have split the dry satchet into two batches?
A dry sachet contains about 200 billion cells when new, with a decrease of around 4% per month thereafer.
For a beer in the 1.070 range, recommended pitching rates are 1-1.5 million cells / ml / 4 SG or about 330-450 billion cells in 5 gallons. So pitching a single pack of 200 billion cells is going to be underpitching, and one pack split is very much underpitching - you're getting a maximum of 100 billion cells compared to the 330 billion minimum needed.
You can make a perfectly good starter with dry yeast, just as with liquid yeast. The only reason most people don't make a starter with dry yeast is that it's inexpensive enough to usually just buy an extra sachet. But in your case, if that's not an option there's no harm done in making a starter. In fact, I would recommend it, and would split the dry yeast into 2 2-liter starters to ensure you reach the cell counts required.
Lets put aside the dry yeast starter issues for now...
I don't think one packet into only 2L of starter wort will be enough yeast for 2 five gallon batches. You probably didn't get much growth (if any) from a dry packet in only 2L.
The better plan is to pitch the starter into one beer. Then brew the next beer in two weeks time.