The merge block is not used to combine signals - it is used to select one of two triggered signals. The two inputs are signals that are conditionally on (such as the output of a triggered subsystem) and only one of the two signals can be active at any time. The output of the merge block is the active signal.
To "combine" the two into one signal, you will need to be more specific on what kind of signal "combination" you want, since there are many types of signal tansformations available.
