You don't need two swap files.
Each install can use the same swap file.
The boot flag is meaningless, it does not matter if it is on or off.
All you have to do is to select manual partitioning, then choose the partition that you want to over-write and set it as /.
Secondly, you can either pick the Ubuntu swap file ( which will probably format it and change the UUID so you will have to edit your Ubuntu /etc/fstab file,) or you can install with no swap partition and later you can add the line concerning the swap partition from the Ubuntu /etc/fstab to your new install's /etc/fstab.
You can certainly delete the other swap file and merge it into one of your partitions.