klalido - I would create a mosaic dataset in your geodatabase and point to the 2 *.tifs to mosaic together. This will get around the display issues you're experiencing.
thank you for your kind response, this has fixed the display issue but the problem is that the classes are gone and turned into 3 bands image again. I need those classes for analysis because they represent land use/cover, is there a way to merge them and keep the classes?
Another option to consider is the Mosaic geoprocessing tool or maybe the Mosaic to New Raster GP tool. Explore the parameters to ensure that the output raster dataset colormaps match the input images.
I used the reclassification tool first to reclassify every raster and then merged them using the merge tool in the raster function. This seems to do the trick! Thank you again, sir. have a wonderful day