I think I can help on this one since I made the same mistake in thinking BUT didn't have a Forum to ask before doing. I had Poohbear who was a whwh Satin ee Melanistic yellow male who was SH Ll. I found a beautiful Satin LH REC Roan girl who had and outstanding coat.
The only male I had which was whwh was Poohbear so I thought the exact same as you that Sasa was just as good as SaSa. Well the resulting litter proved how wrong I was in this thinking as the pups with double satin genes were awful coated and needed extra care their whole lives to prevent them from getting chilled and going into Hibernation. Constant care with their skin getting cuts and even with excellent care getting infected.
There were 13 pups in that litter and two ended up SaSa and took a long,long time to place in homes as they were just plain ugly. Their coats were so sparse it made them look like they had mange or some kind of illness which caused major hair loss. Of course it was only their hair follicles were double satin and that caused the fur to be very very thin. One new owner LOVED her boy cause he was always much warmer to the touch and he liked to lay on her shoulders which were arthritic. His warmer skin would actually sooth her aches and pains when her Hamster laid on her shoulders.
However along with that luxury came many many hardships too. Like hairless they were very prone to skin cuts and sores which for some reason were easily infected. Winter in a colder climate meant heat lamps or heat pads 24/7 which made the electric bill go up. Both pups had to be bedded with paper bedding and required daily brushing to prevent major hair mats on them even though they were SH. Aspen chips would stick to their coats like it was glued on them which would cause cuts on their skin if not combed or brushed out daily.
I will NEVER breed Satin X Satin ever again as it is like deliberately breeding two Wh mates and just saying "Oh well I'll just deal with the Anophthalmic pups if they occur.
Now Rex to Rex I had seen the eyelashes curl under and cause major health issues with the eyes being torn from the inverted eyelash. Major health issues should be avoided at all cost when you are a responsible breeder. This inverted eyelash problem only occurred when this well known breeder bred two Rex parents together and never when one was a rex and the other a rex carrier.
However why would you want to risk producing a pup who is unhealthy and will need high maintenance, when you can get the same looks by using one satin parent, and one normal parent to produce beautiful satin coated hamsters, and one rex and one rex carrier to safely avoid any pups who will not live without major surgery or have eye issues all it's life?
Here's some pics of the parents and the pups of my Sasa XSasa litter the Chedderbears
The Sire Poohbear
Poohbear indoors-a.JPG
The Dam Chitterchedder
Cheddarbears-day 24-Chitter Cheddar-a.JPG
Here is a Sasa LH BEC Roan male pup at 24 days old
Cheddarbears-day 24-BEC Roan LH Boy#1-c.JPG
and a SaSa SH BEC Sister at 24 days old Note how you can easily see her pink skin through her very thin coat as like an animal with full blown Mange.
Cheddarbears-day 24-Cream SH girl #1.JPG
and see if you can spot the two SaSa pups among the Sasa siblings in this suppertime photo(Both SaSa pups are readily visible
Cheddarbears-day 32-suppertime-c.JPG
Hope this helped to shed light on why to avoid producing double satin pups and why it may be and issue with health to breed two Rex coat hamsters together as well.