Cant read HDD

Hey guys.
I just bought a new PC with a new harddisk.
My old PC had 2 HDD that were used as a master and a slave to form 1 partition.
I didnt wanne lose the data from my old harddisks so i putted both in my new PC aswell.
I installed a fresh Windows on my new main drive and just wanted to use the other 2 old HDD as dummy's just 2 leach the data from.

But now the main problem.
I cant enter the old HDD anymore. I can see one drive as a disk station but cant see the other one.
If i look at the BIOS i do see both and in windows diskmanager i also see them both online.
But they aint connected anymore as 1 disk and i cant enter both disks. Also if i look at the properties it doesnt show the disk size.
If i wanne convert them together again he says that there isnt enough diskspace on them.
I dont wanne lose the data ofc so dont wanne format it.
Maybe there is a way to make them 2 sepperated disks again without losing the data?

Anyone with an idea?

Aye sorry forgot to say.
I made them a RAID 0 as NTSF ofc.
Now it shows the only drive i can see as a RAW disc instead of a NTSF.
The other just shows as "Not assigned"
And nope, i cant assign it.

I i put them back in my old PC it doesnt boot. It says: "cant find RAID 0, failed to boot"
For some reason both drives just lost the connection 2 eachother.

You probably already corrupted your data by trying to 'convert' them or reinitialize them.

You can try RAID recovery software or you can try linux to rebuild the RAID0. In either case, should you have written to the two disks, you're probably facing data loss. Without any tampering, full recovery should be possible but not when writes have reached the HDD; then information is lost you can't recover anymore.

Ye i make backups every 2 months or so.
But still lost some good stuff.

O well ill try the recovery tool.
Thanks for the help guys!
Next time ill build a raid 4 or 5 atleast its save.