How can I switch between two voltage source connected to the same node for some
time and duration using HSPICE elements.

Answer:

There are two ways to do this.  One is to design a Verilog-A module that can be
used as a behavioral switch.  Another is to use the HSPICE voltage controlled
voltage source (E-element) to design a behavioral switch.

The netlist is as follows:
***************************
.option post
Vin1 in1 0 pwl ...        $ source 1
Vin2 in2 0 pwl ...        $ source 2
Ein in 0 vol='v(in1)*v(ctrl) + v(in2)*(1-v(ctrl))'    $ behavioral switch
vctrl ctrl 0 pwl 0 1 49n 1 50n 0         $ control voltage
.tran 1n 100n
.end
****************************

In the above example, v(ctrl) is set to 1 initially so that v(in)=v(in1).  At
50nS, the control voltage is set to 0 and v(in)=v(in2).