You can just install one SSD and install an operating system to it. Then install just the second SSD and install the second to that.
Installing Ubuntu Alongside a Pre-Installed Windows with UEFI
Not sure what type of RAID you want that will easily work with both Windows & Ubuntu. I do not know RAID, but prefer to have the drives separate and do my own backup to the other drive which some archiving. With RAID there is no backup, just duplication.
I would suggest gpt partitioning for all drives. But Windows only boots from gpt drives with UEFI. Ubuntu can boot from gpt drives with UEFI or BIOS, but you really need both systems installed in the same boot mode to easily dual boot.
If a 256GB SSD, that would be very large for / (root). I have a 60GB SSD and have two different / including /home inside root, so user settings are also on fast drive. But all data in in rotating drive and linked back to /home on SSD. My /home then is mostly the hidden settings and any normally hidden folder with more data like Firefox profile also gets moved to data partition on rotating drive.
For the Total space you want for Ubuntu:
Ubuntu's standard install is just / (root) & swap, but it is better to add another partition for /home if allocating over 30GB.:
Only if gpt - all partitions in gpt are primary add these two first:
gpt: 300 MB efi FAT32 w/boot flag (for UEFI boot or future use for UEFI, you only can have one per drive, so if already existing do not attempt another)
gpt: 1 MB No Format w/bios_grub flag (for BIOS boot not required for UEFI)
for gpt(GUID) or MBR(msdos) partitioning
Ubuntu partitions - smaller root only where hard drive space is limited.
If total space less than about 30GB just use / not separate /home or standard install.
1. 10-25 GB Mountpoint / primary or logical beginning ext4(or ext3)
2. all but 2 GB Mountpoint /home logical beginning ext4(or ext3) or inside / and separate data partition(s) on rotating drive.
3. 2 GB Mountpoint swap logical
Depending on how much memory you have you may not absolutely need swap but having some is still recommended. I do not hibernate (boots fast enough for me) but if hibernating then you need swap equal to RAM in GiB not GB. And if dual booting with windows a shared NTFS partition is also recommended. But you usually cannot create that as part of the install, just leave some space. Or partition in advance (recommended).
One advantage of partitioning in advance is that the installer will use the swap space to speed up the install. Thanks Herman for the tip.