Sometime back (a few years now I think) I bought an HF refrigeration
type air dryer off of Ebay. When the item didn't show up when it should
I contacted the seller. Again now show so I emailed him. It went on
like that for a few days. He never emailed or messaged me back to start
with. Then all of a sudden I wound up with four of them. I only paid
for one.

They all seemed to ship from a warehouse in Chicago that I think is
owned by Harbor Freight. He may have been an agent resellers for HF and
didn't have direct control of the inventory.

I suppose I could have been dishonest and just kept them, but I messaged
the sellers and asked him why I now had four (4) air dryers. He asked
me to ship them back, but my cost for shipping would have been
exorbitant. I asked about a deal to keep one to have on the shelf as a
spare. He made me a smoking deal so I kept all of them and paid him.

I then resold two of them and more than the cost of the two I kept. It
turned out Harbor Freight didn't have any more (at the time) and the
guys who bought them had already tried Harbor Freight. One drove over
to Yuma from Mes, and the other came from Lake Havasu, and both were
thankful to get them.

Well, now some years later my spare is still in the box on the shelf,
aad the original is still working just fine. I did put in on a stand
and drill a hole int eh bottom of the case so the drain line wouldn't
act like a trap sometimes keeping the internal separator from draining.

I think I may be reaching the capacity of the dryer. Particularly on
humid days. No I haven't seen any moisture in downstream separators
(one at every machine), but it just feels like it. The only time I had
a moisture problem was when a 3 coupling block blew out of a manifold
and the system ran at capacity for the whole time I was eating lunch in
the house. It took me two days to get the moisture out of the lines and
equipment to my satisfaction. Anyway, it works, but I may be near
capacity. I am getting ready to add another spindle with an air seal,
but that doesn't use as much air as you might think. The big deal is I
am seriously considering adding a permanent air blast on the machine I
use most of the time for milling steel.

ALL of the preceding information was background. I'm not asking for
validations on any of it. LOL.

Here is the quandry. Could I double my dry air capacity (pretty sure I
could) by just tying in my spare air dryer in parallel with the first?
Sure thee would be double regulators and double other hardware and
plumbing at the compressor, but what other drawbacks might there be?

I currently close a valve at the tank drain the lines, and shut off my
air dryer every day at the end of the day. This allows it to fully
drain, although it periodically drains when in use.