Is there any way to mount 2 standard SATA hard drives in a 5.25 drive bay?
Maybe each 3.5 sideways?
I could run the SATA data and power cables through the bay?
There are none that will give you 2-to-1.
except this one: http://eshop.macsales.com/item/OWC/MM52T35/
Sure there is , like a Akust 5.25" Bay to 3.5" HDD Adapter / Noise Reducer - Black (AV00-0100-AKS)
The Akust 5.25" Bay to 3.5" HDD Adapter / Noise Reducer allows you to mount up to two 3.5" HDDs in a single 5.25" bay slot
Remember that positioning hard drives flush against each other can cause heating (and magnetism?) problems.
That said, it depends on what you mean by "sideways".
Much of these questions depends on the actual design of the bay.
3.5" HDDs are actually too big to fit two of them into the 5.25" drive bay (the sum of sizes should tell you that).
Not to mention the heat issues: two drives mounted so close together would have serious heat propagation issues and would be running at noticeably bigger temperature, thus increasing chance of failure dramatically.
All in all, it's a bad idea.
If you want to place just one 3.5" drive in a 5.25" bay, buy some rubber hard disk stabilizers like the following
For instance if you want to achieve a higher "3.5 to 5.25" ratio, you need some some boxes like:
or
which gives 3-to-2 or 4-to-3. There are none that will give you 2-to-1.
Even if you should manage to pack two disks together in a 5.25" bay manually (don't know if there is physical space enough) I would strongly advise not to. Having enough ventilation is important, you do not want your hard disks to get too hot.