Do dell perc 6i cards support online expansion of a raid 1 array when both drives have been replaced with larger ones?

That doesn't seem possible in PERC 6/i controllers.

You could break the RAID-1 volume, replace the drive with a bigger one and create a RAID-0 volume on it. Migrate the data, do the software configuration and replace the remaining disk. Not as easy as online expansion though.

You don't mention which software you use but if it is ZFS, you could follow the suggestion above and attach the new RAID-0 volume to the existing disk. Later you destroy the degraded RAID-1 volume, create a RAID-0 on top of the new disk and attach it to the disk. It's very easy.

From what I`ve derived from James comment it seems you can use the same method as with any virtual disk: Shut down the OS, resize the disk without changing vital parameters (the geometry of the vdisk and stripe size have to stay the same). So if your vdisk geometry after the new size has just more cylinders it should work.

But - this is offline, of course.