This PDF File teaches you how to use the E3 flasher to do the dual boot Switch the PS3 Firmware between PS3 V3.55 and PS3 V3.40
More Details check here:
http://www.digitopz.com/e3-flasher-downgrade-ps3-v370-to-v355-p-1091.html
More Details check here:
http://www.digitopz.com/e3-flasher-downgrade-ps3-v370-to-v355-p-1091.html