As stated you can not put the same DN on the phone more than one time, in the same partition. But you can allow up to a maximum of 200 calls to the same DN, so there is no real need to have the other line with the same DN.
I'm guessing you are referring to a Callmanager on the 3.x train? If that is the case, here is the only method to alow Multi-call to a single DN on the same phone;
Cisco CallManager 3.x does not have a quick and easy way to accomplish this. Until the ability to have more than two calls on a single line appearance is available (CCM 4.x), this document shows how to use Partitions and Calling Search Spaces to configure multiple lines on one phone with the same DN.
From this doc;
Configuring a Single Dialed Number on Multiple Lines
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If you have 2 ISR routers, one acting as Failover, do we need to have both the same number of SRST licenses on the 2 routers?
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