This would be out of the box functionality in Network Configuration Manager (NCM)
Okay. Could you be a little more helpful? Which report is it? I looked through the default reports and I don't see one which allows me to compare the running configs of two routers.
Going on a few assumptions here, that you have a recent version of NPM/NCM and that your question is pending.
Step 1. Navigate to the Solarwinds website and hover over 'My Dashboards'
Step 2. Hover over 'Configs' and select the 'Configuration Management' option.
Step 3: Search for the routers in question, once identified select them both by checking the box to the left.
Step 4. Select 'Compare Node(s) Configs' and then select 'Compare Node(s) Configs' again. Once you make this selection Solarwinds will compare the configs for you based on the date of the config you select.