The idea is not to use the same single home directory for multiple distro. The idea is to use a common, shared, separate data partition for all of them.
Each distro should have its own home. It may reside under the root. Home holds a lot of distro-specific settings, configs, caches, etc. Mixing home-s from different distros is probably worse than mixing two DEs in a single partition.
The data partition is not provided by default by any of installers. A user should add it by hand. Either during installation, if an installer allows to add user-defined partitions, or once installation is completed, if an installer doesn't allow to use non-standard partitions.
My computers run from 13 to 26 different, independent Linux-es (DEs). Each Linux (a single DE) is installed in its own, separate partition, with its own home. Each Linux partition is small, only 20 G in size.
But each computer has one common for all installed Linux-es and shared between them data partition. It varies from 500 G to 2 T in size. Enough to hold my data files, accessible at any time from any Linux. There are no config or settings files here. It's not a "home".
Beside data files, this huge data partition holds also various art files. These are Gtk2, Gtk3, shell, Plasma themes, icon and cursor themes, color themes, sound themes, fonts. There's no need to waste disk space and install the same theme several times in different DEs. It's enough to soft-link it from data partition to any Linux. No space wasted in root partitions to have tons of preferred themes everywhere.
I use this schema for more than 15 years without any trouble. A common shared data partition is a great thing.
Back to topic. How much space is needed for Linux partition (one partition - one DE)? There's a simple rule of thumb here. To feel itself in good health and to breathe freely, Linux should have at least 25% of free disk space on startup.
In other words, static system and user files should occupy not more than 75% of a partition. 25 remaining per cent are used by Linux for temporary files at run time.
A "normal" DE takes from 5 to 15 G of disk space, including config and cache files in home folder. 20 G partition (one partition - one DE) is usually enough for an average Linux.