The way I do wudhu is that I pour an amount of water that I feel is necessary or sufficient on the respective part, and then wipe it until I am sure all areas have been covered.
If for eg. while wiping my left arm with my right hand, my left hand touches another surface that is also wet such as the sink (or water tap to stop the water supply etc), does this mean that it has mixed with a "new" different source of water, and if my right hand wipes over or touches this area on the left hand then the wetness on my right hand will also be mixed with "new" water? And therefore I won't be able to do the wiping of the head and feet? Or can I just disregard the fact that my left hand touched a wet surface, wipe over it with my right hand and continue as normal?
Apologies if the question is not clear, it can be rephased if necessary.
When washing your arms, your hands can touch a wet surface. After you have fully washed your left arm, make sure not to acquire new moisture. So before you finish washing your left arm, shut off the water and continue the wudhu.