To get you in the right direction add this (of course adapt it to your needs first) in your fist MSI:
<DirectoryRef Id="INSTALLDIR">
<Component Id="RegistryEntries" Guid="{0AC76129-F8E2-47D3-B9FD-09B1E10A8541}">
<RegistryKey Root="HKLM" Key="Software\Company123\App123" Action="create">
<RegistryValue Type="integer" Name="SomeIntegerValue" Value="1" KeyPath="yes"/>
<RegistryValue Type="string" Name="UserInput" Value="[USERINPUT]" />
</RegistryKey>
</Component>
</DirectoryRef>
Don't forget reference the component in your <Feature>
<ComponentRef Id="RegistryEntries" />
When you install assign a value to the property [USERINPUT]
e.g. msiexec /i your.msi /qb+ USERINPUT="the value to be saved in registry"
Then in the second MSI add something like this:
<Property Id="READREGISTRY">
<RegistrySearch Id="USERINPUT_Value" Root="HKLM" Key="Software\Company123\App123" Name="UserInput" Type="raw" />
</Property>
The value/string you entered during installation USERINPUT=
will be stored in your second MSI in the property READREGISTRY
Here a piece of log in my second msi:
PROPERTY CHANGE: Adding READREGISTRY property. Its value is 'testing registry wef wef wef w'.
Based on your installation where it might be Per User or Per Machine you may adjust the Root to HKCU for PerUser installs or leave it to HKLM for PerMachine.
For more information please refer to Wix documentation, hints: "How To: Write a Registry Entry During Installation" and "How To: Read a Registry Entry During Installation".