Could be for Rake maintenance. It might be that the primary maintenance for both trains is at Delhi. So, the cycle will be like DLI (primary maintenance) -> JU ( as Mandore) -> BDTS (as Suryanagari) -> JU (return) -> DLI (as Mandore) -> Primary maintenance. However, I am not sure as to where the base maintenance of the rake is. Someone should confirm this.
This is a common practice in many places. Examples are 17644 (COA-MS) and 17651 (KCG-MS). Both reach Chennai Egmore in the morning and leave in the evening within 20 min of each other. However, the base maintenance is at KCG, so there is an RSA between them. Another example is 12164 (MS-DR) and 11063 (SA-MS).