How you go forward will depend upon what software you are using, how your data is set up, and what you are trying to achieve.
So, to help out responders:
1. What software do you have available? ArcMap? And what Version?
2. Do you have access to any extensions? Specifically Spatial Analyst or 3D Analyst?
3. Do your tiffs have a coordinate reference system (crs) assigned?
4. Are your tiffs located spatially next to each other and you want to combine them into one? Or instead are they overlapping and you want to come up with an output tiff that combines the values of the two originals?
5. Do your tiffs have similar value ranges or are they vastly different? For example, one has a range of zero to ten, while other has a range of zero to 255 - that would make them different.
If you can answer these questions, that would help greatly in providing answers that will be useful.
Chris Donohue, GISP