My recommendation would be to utilize LVM. Set up a PV on the new drive and create an LV filling the drive (wit an FS), then move all the data off of one drive onto this new drive, reformat the first old drive as a second PV in the volume group, and expand the size of the LV. Repeat the process for the second old drive. Then, instead of extending the LV, set the parity option on the LV to 1. You can add further disks, increasing the LV size or adding parity or mirroring in the future, as needed. This also gives you the advantage that you can (once you have some free space) create another LV that has different mirroring or parity requirements.