Can I use a 3500mAh battery with a 3400mAh battery?
No, it is always best to use batteries that have the same capacity as one another when operating a flashlight that requires more than one battery.
The mAh, or milliamp, level of the battery is the indication of its capacity. When a flashlight uses multiple batteries, it pulls and equal amount of energy from each battery at the same time.
If you have two batteries that can hold a different amount of energy, one may become empty after 1 hour whereas the other might become empty at 1.5 hours. This means that even if the first battery is completely dead after the first hour, the flashlight may still detect that the second battery has energy in it and continue to try to drain energy from both batteries. This can cause many problems. The weaker 18650 can be over-drained and reach a state of "deep discharge" which will kill the battery permanently. The flashlight may try to make up for the power it is not receiving from the weaker battery and put more strain on the stronger batter. Or, the flashlight circuit may not know how to balance the lack of energy being received and could cause internal circuitry issues.
Learn more about 18650 lithium-ion batteries.