Extending capacity of small air compressor

Overall run time, no. What it will do is extend the time period until the compressor kicks on the first time. After it kicks on the first time, it'll have to re-fill both tanks, so it will run longer.

Plus you need to understand that the actual "run time" when the compressor kicks on will double, triple, etc. depending on just how much larger your new tank configuration will be. You may want to consider that you will be asking that compressor motor to possibly work much harder than it may have been designed to run. In other words, extended run times will shorten the life of your compressor.

I have done this and am currently using a small Hitachi compressor tied to a 17 gallon tank that used to be a Lowes compressor that I cannot get parts for! It definitely does take longer to fill BUT, as jmartel says above, it takes a lot longer to get to the point when it needs a refill.
This has worked very well for me and I am glad I did this!
The tanks are tied together with an open air hose.
Good luck with yours!

I would be worried about exceeding the duty cycle of the compressor. You mentioned it was a "small" compressor, if adding a 5 gallon tank to a 1 or 2 gallon compressor I would certainly recommend against that. If it's a 20-30 gallon compressor, an extra 5 gallons wouldn't represent nearly as much extra work on the motor and pump.