If you just want to add them together, you can do that very easily with Gromacs:

gmx trjcat -f traj1.xtc traj2.xtc (also works for .trr as far as I remember)

If there are overlapping timestamps in the trajectory, you may want to use the -cat option to retain all frames.

http://manual.gromacs.org/current/programs/gmx-trjcat.html

If you just want to add them together, you can do that very easily with Gromacs:

gmx trjcat -f traj1.xtc traj2.xtc (also works for .trr as far as I remember)

If there are overlapping timestamps in the trajectory, you may want to use the -cat option to retain all frames.

http://manual.gromacs.org/current/programs/gmx-trjcat.html

Adding to the above answer, you may need to align, unwrap, or wrap the trajectory (particularly for membrane simulations). 

The second part of the below link discusses the difficulty of this.

http://www.gromacs.org/Documentation/Terminology/Periodic_Boundary_Conditions

You can also add to your already loaded molecule in VMD by right clicking on the loaded  model.

First do "trjconv"   to convert .trr file to .xtc and then try "trjcat" to concatenate both xtc files into single xtc. Use -settime option in trjcat command.