Another option is to add a bit more grain for your second runnings. There's no reason you must keep the grain bill identical. For example, you could take a bunch of two-row to make a barleywine... and then add some crystal malt and make an amber... or some roasted barley to make a stout. Your second beer can be darker than your first with a little extra grain.
Although this isn't historically accurate, you can also cheat. If you make a decent-sized barleywine, and follow that by a pale ale using 3lbs of DME added in, you can get a big beer and a normal-sized beer, for the price of a couple pounds of DME. If you wish to use more historical methods, you can also do a 3-5 hour boil on your second runnings to increase the gravity. But sometimes, it's worth it to just throw a little malt extract in there instead.
I have also added more two row into a mash that's already had the first runnings run through. You lose a little bit of efficiency, but you don't need more than a couple pounds of grain to get a normal-strength second beer. Just remember that if you do this, you have to wait for the new grains to convert. And if you pull too much out using this technique, you may get some tannins from the husks of the grain.