It is possible to place several related product blocks in one placeholder on the webpage. However, this functionality has some limitations connected to the native Magento placeholders, and requires correct configuration in order to work as intended.
In order to correctly place several blocks in one placeholder, it is required to disable native Magento upsell, cross-sell, and related products blocks. This can be done via the Magento backend admin panel on the module's configuration page. When several blocks are placed into one native placeholder, only one block will be displayed.
While it is not possible to insert two related blocks into the same native block, this can be done for other positions that a page provides. Use the placeholder Product Page | Content Bottom to achieve the desired result. See more on the related products extension magento demo