Water quality is most important to an axolotl's health. Could you advise us as to your water parameters (Ammonia/Nitrite/Nitrate/PH and Water Temperature)?
Also, what are you feeding the axolotls? If both axolotls are losing pigment in their tails but are in seperate tanks then the food is something they both have in common and is worth looking at - it may be caused by some sort of deficiency in their diet? (I'm making this a question as I don't know if this would be a cause or not).
What size are the tanks?
These details will give us something to work with to find out what could be up with your axies.