Is there any way I can join two iso files into one and burn a dvd ?
The isos in question maybe linux distros or bsd install disk or anything else ..

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You have two files which were originally one file which has been split in two for some reason, or you want to make a multi-boot DVD from separate CD images?

You have to mount the two iso files, copy everything they contain into the structure you need, then convert that (third) structure back to an iso for burning.

If you just want to store a number of distro iso's on a single dvd, it is normal just to burn the raw iso's as files. You can fit many iso's on a single dvd this way. When you want to install, you copy the iso to a CD to boot from.

Otherwise you have boot issues trying to use the combined iso of many distros to install a particular nix.

If you have (say) video on two (or more) CDs that you'd like to turn into one long continuous movie, the best way is to copy the files and splice them together with your favorite editing tool.