It doesn't matter which is installed first, both windows and mac versions are on the cd (There is no prime version). You will have to read the license agreement about use on your machine then on your clients. The software will run standalone on two different machines and just don't know if the license agreement will allow.
If you would have bought before Christmas you could have got two copies for the price of one, maybe they will have another sales again soon.
You will have to ask your client if they perfer download or disc. Most companies hirer out because they don't want to make those decision, they want an all in one solution. I know some clients can't keep up with disc, somehow they magically disappear.
Whomever owns the software, is whom the registration details name should be in.
You can alway get a 30 day FMP trail to test your app.
The install files from a download or the CD can be copied onto a Flash Drive.
If the drive is USB 3 and the computer has USB 3 - it is a much faster install and a very secure backup (no scratches on a flash drive)
However you store the installer, Make more than one copy and store in more than one location. Give one to the client and ask them to drop it into a safe deposit box or something equally secure and off site. Put any Full access passwords that you've delivered to them in a text file on the same media.
Pity about the sale - I missed that. But I will very likely be buying Advanced for myself, if anyone else asks me to write them a database. This isn't my 'real' job, though I do like the idea!
Thanks for the advice above. I'm going to go for the download + discs option, so I can give the client something tangible. And will make backups and think about security as suggested above.
PS - Having paid and received download link, I had to choose Mac or PC, so I'm glad I ordered the DVD too, as I wouldn't be able to install the Mac version on a PC. That would have been a disaster...