I would put the first batch in a sealed and sanitized bucket in the fridge and put them in the fermentor at the the same time. That way I could get an accurate OG reading.
You can do it the way suggest. Your first batch would be one heck of a yeast starter.
You can also just take the weighted average of the two OG readings and get to what should be the same number. It should be pretty straight forward. Assuming both batches are the same volume you wouldn't even have to weight the average.
I wouldn't want to wait to pitch the yeast. I'd do just what you (the OP) are proposing.