Is it possible to expand a ReadyNAS by connecting a second one to it

To answer your specific questions:

I don't know if the 524x supports multi-drive eSATA units that use SATA port multipliers (like the discontinued EDA500) or not.  Most of the newer ones don't, as it turned out to not work out very well (which is the likely reason the EDA500 was discontinued).  I don't recommend you even think about it unless it's for archival data that's rarely accessed.  Netgear has not added support for multi-drive volumes in a multi-drive USB unit.  Any USB box with it's own internal RAID should be able to be used by the NAS; but the OS wouldn't see the drives separately, it would see one big USB drive.  I tried that with a Drobo USB3 DAS unit just for kicks, and it worked.  But Drobo has it's own issues and you really don't want to run one on a system where you can't use their management tool, so I don't recommend that specific brand, either, and any that require a tool that can't be run on a Linux system are probably best avoided.

You cannot hook two NAS together as master-slave.  You can mount a share on one NAS on another, but that doesn't really meld them together and you're bucket-brigading the data from one through the other, so it makes little sense in most situations.  Maybe if you don't use 10GB for your network and you connected two via the 10GBE ports, it would.  But you'd still need direct network access to the second one.

I've not tried it, but you could probably create a volume on a multi-drive USB unit using Linux commands via SSH.  But it would be best to try it on a system where there is no critical data, in case it brings it all crashing down.  Expanding an existing volume to an external box would be a really bad idea, though probably also possible via SSH.