They overlap in many areas in different ways. Furthermore, SysHardener disables some functionality of H_C. For example, both SysHardener and Hard_Configurator prevent the user from running files with dangerous extensions like: VBS, VBE, JS, JSE, WSF, WSH, HTA, SCR, PIF, etc. Syshardener can block 13 file extensions (by file associations) and H_C blocks 42 (by SRP, including all blocked by SysHardener) + additional added by the user. H_C allows to whitelist some blocked files, but SysHardener will block them anyway. H_C allows running those files from the elevated shell, but SysHardener will block them anyway.
Suppose that the user wants to switch off temporarily the H_C protection and run some blocked files. Then files like MSC, CPL, CHM and many others could be opened, but VBE, JS, JSE, WSF, WSH, HTA, SCR, PIF would be still blocked.
If the user turns OFF SysHardener protection, and keep H_C protection, then he/she does not notice any difference, because all dangerous file extensions will be still blocked by H_C.
The problems can happen also with unblocking remote features, PowerShell Constrained Language mode, hardening settings for MS Office and Adobe Acrobat Reader, etc.
Generally, in the home environment on the updated Windows 10 with updated software, SysHardener is redundant when using H_C in Recommended Enhanced settings. In the upcomming H_C version, Recommended Enhanced settings will block more Sponsors:
powershell.exe, powershell_ise.exe, bitsadmin.exe, csc.exe, hh.exe, ieexec.exe, iexplore.exe, installutil.exe, msbuild.exe, mshta.exe, presentationhost.exe, reg.exe, regini.exe, regsvcs.exe, wmic.exe, bash.exe, regasm*, schtasks.exe, scrcons.exe, windbg.exe.
If one wants to use the SysHardener firewall rules for LOLBins, then it is OK, but it is safer to block them in H_C.
There would be some advantage of using both H_C and SysHardener in the enterprise environment, because some SysHardener settings can block processes running with Administrator rights. In H_C, most restrictions blocks (by design) only processes running with standard rights.