Really long way around, but doable.
In the future, you could just set up a profile for no sparge in beersmith.
You can over build your recipe, so instead of 2.5, go for 3 or 3.5g.
Collect wort until you hit your preboil volume and close the valve. Drain and sparge accordingly for the second beer.
You can estimate the second beer volume by adding the volume to your no sparge(add it to sparge volume, and adjust recipe volume) and calculate/estimate the difference.
The important thing is making sure you get the OG of the first beer correct. Having extra wort ensures that you'll have body and a decent amount of sugars for the second beer.
I've always looked at the second beer as not as important as the first.
The big thing for me in doing it this way is I am still locked into my equipment profile, losses, temperature variables etc.
As it stands, you could setup a no sparge profile, build the recipe as needed for beer one and brew it no sparge. Following lauter, you could add your additional grain and do essentially a second mash with new and old grain. It you were going to have a gap in time anyway, no time lost there. You may take a bit of an efficiency hit in doing so, but at your volumes, we aren't talking much cost wise.
I hope this makes sense. It pays to be more calculated and have less surprises.
As your current plan stands, I would encourage you to collect most of your wort before checking gravity for dilution and check the gravity from blended wort in the kettle. The gravity from the first runnings is not consistent and may vary a bit as the tun reaches the second half of the runnings. If you judge by the very first runnings alone, you may undershoot your OG.
*Not trying to criticize here, by any means. There's 100 ways to get to Detroit.*
But, you'll always get more bang for your buck by sending all water involved in your beer through the grainbed. Dilution should really only be used if it is to overcome an equipment restriction or to correct a high gravity issue.</opinion>
Annnooottthhheeerrr option, as it stands, is punch your recipe in now, including all grain, set no sparge, then adjust your water volume until you hit your target OG. Mash, fully drain. Whatever wort is of excess is combined with sparge for the second beer.
Also(last thing, I swear), don't be afraid to oversparge a bit and extend your boil to increase your overall efficiency, especially if you over collect on the first lauter. I wouldn't do so on smaller beers to much though. Sorry if something above doesn't make sense and I probably left something out. I'm pretty exhausted.
Cheers!