You should definitely not use striping (RAID 0) or any more advanced RAID level (like RAID 5) with the ESP. At first glance, you might think that RAID 1 (mirroring) should be OK; however, if something outside of Linux (like an EFI boot loader) were to write to the partition, that would result in inconsistencies that would cause problems. It'd be hard to rule out the possibility of something like that happening. Thus, I'd be reluctant to use RAID 1 on an ESP. If you want some redundancy, you could create a second ESP on your second disk and manually sync them every now and then -- perhaps even automatically via a cron job or a startup script.
All of this assumes Linux software RAID, of course. If you've got a hardware RAID card, the EFI will see the RAIDed drives as a single device, so there's no issue. With a motherboard-based software RAID, you'd need to check to see if the firmware understands the RAID setup.