The custom templates are stored in the project configuration files that's why it's not shared between projects.

There's no option in the UI to share those templates. The only unofficial way would be to manually edit the "config.zip" file at the root folder of your project, edit the "repository.xml" file and go to the end of that file. You will see a custom templates section.

If you look at the project where you have your templates already created, you just have to copy the whole section and paste it into the other project configuration file at the exact same place.

Before doing that i would recommend you to backup your project folder in case you make a mistake since it can corrupt your project and remove some files that won't be recoverable later on.

Here is an example of the template section with one Group:

<charts-templates-description name="template.customs">
    <charts-model-description name="Group_0">
        <chart-description autoRange="true"
            chart-description-type="chart-description-type"
            domainMarkerVisible="true" granularityLevel="0"
            historyState="FULL" includeInReport="false"
            rangeMarkerVisible="true" title="Controller/User Load" typeName="CLASSIC">
            <monitor-curve-description
                benchUid="dcfb2952-294e-4e44-afd1-babb8d8599b3"
                color="20,106,203" connectionName="NeoLoad"
                dashed="false" monitorName="UserLoad"
                monitoredHostName="NeoLoad" scale="1.0" selected="false"/>
            <monitor-curve-description
                benchUid="dcfb2952-294e-4e44-afd1-babb8d8599b3"
                color="255,200,0" connectionName="NeoLoad"
                dashed="false" monitorName="memory"
                monitoredHostName="NeoLoad" scale="1.0" selected="true"/>
        </chart-description>
    </charts-model-description>
</charts-templates-description>