We have just taken delivery of a pair of PIX 515's to set up as a failover pair, one has been shipped without software - I need to get these up and running asap. Is it possible to copy the software of the good device and install it onto the other? If so how would I go about it?
then on the other you would go into the ROM monitor at boot time and copy the software in from the tftp server.
However, new PIX are always shipped with software (unless you pay a fair bit for a PIX chassis as a replacement part). This and the fact that you name 515 rather than 515E suggests that your PIX are used. If so, then unless you took extra precautions, then the software license did not transfer to you and you likely do not have the right to run either PIX, let alone as a failover pair (which requires Unrestricted license for one member and an Unrestricted or FO license for the other member.) A device that arrives without software is likely not to have stored a license key for what you want to do.
If the device is used and you did not get it from a vendor specially authorized by Cisco to sell used equipment (and those vendors would always include the software), then you need to "relicense" the software. The part numbers are LL-PIX-515-SW-FO and LL-PIX-515-SW-UR; the latter appears to be selling for about $US4200 when you can find it in stock.