[howdy20012002]

If you don't quarantine, by the time you notice something wrong with your fish, your entire system will most likely be infected.
turning the second tank off from the system will not work as it will already be too late. Therefore, there is absolutely no point in having a second tank attached to your main system as a quarantine. Parasites will swim around the water, find the overflow and be in the other tank in no time.

appearance of a fish means nothing...fish can show absolutely no signs of illness and then be dead within 48 hours.
Fish that die from marine velvet often don't even show signs.
those that do show signs, by the time they do it is often too late for treatment unless you agressively treat with copper.

The store could have the fish for months..but what else have they recently added to the same system.
there are too many variables for you to know the status of fish that you are buying out of most fish stores unless they have separated the fish and quarantined them for at least 3 weeks and not added anything else that hasn't been quarantined.

I have had fish come in...be fine in quarantine for 2 weeks..just about to put them into one of my main system and all of a sudden they have ich.

if you can financially afford to run a quarantine...run it.
realistically, to be completely sure that your fish doesn't have something you miss, you should treat the quarantine with copper or by hypsolanity.
both should be done for at least 4 weeks, if not 6 weeks to make sure that you have a parasite free fish.
otherwise you are taking your chance everytime.