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.

All the previous answers are very good especially if you don't want  to do this using VMD.

With regards to doing this within Vmd GUI application itself. You could use the following steps assuming multiple trajectory files.  1.xtc, 2.xtc, 3.xtc

1) Select  the "New Molecule" pane under "File"  menu in "Vmd Main" window to  load the topology or structure file into Vmd. This could be  .gro  or .pdb

2 optional) On the VMD main window, right click on the loaded molecule of choice, and select "Delete frames..." from the submenu to delete coordinates loaded from pdb or gro file. 

3) On the VMD main window, right click on the molecule, and select "Load Data into Molecule" submenu  to select and  load  1.xtc trajectory into molecule in step 1.

3) Repeat step 3  for all other trajectory filesAdd your answer(2.xtc, 3.xtc ...).

4) After loading all trajectories,  you can save all coordinates in single trajectory file by right clicking on the molecule in the main window, and selecting  "Save Coordinates" to save entire trajectory. Here you select output file type and  then and press the save button.

Hello,

I tried concatenating two xtc files using the command above. But when I do gmxc check for the new xtc file I get this message

Timesteps at t=1.40268e+07 don't match (300, 1200)

How do I fix this?

Thanks in advance