Using 2 7800 series regulators to get -ve, GND and +ve voltage rails
I  have used 2 7815 regs to make a plus minus 15 volt rail for some Opamps .So you could do the same with your 7805 regs to make the plus minus 5V you want .I used a 50HZ mains transformer with separate windings which are common .An advantage of doing this rather than the orthodox CT winding and 7805 7905 pair is better stability on the negative rail .I have found more stability issues from the 79 negative series that the 78 positive series.Better stability gives better  predictability.The 79 regulater is featured in the www.badbeetles.com   website.