There are at least two ways to merge two surfaces
1. Rename the less accurate surface. Paste the more accurate surface into the renamed sufaces.
2. Create a third surface. The first thing you need to do is paste the less accurate of the two surfaces, then paste the second surface into the new surface.
Hope that helps
Merge = Paste
Delete = delete; isolate and use acad delete command
Joseph D. Bouza, P.E. (one of 'THOSE' People) Civil 3D 2012 & 2013
HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
12 GB Ram
Note: Its all Resistentialism, so keep calm and carry on
64 Bit Win7 OS