it would be very nice not to have to do two consecutive resilvers - one for each failing drive. Luckily, ZFS allows you to amortize a single resilver operation over multiple drives.

The workflow is almost identical - you begin by doing a hot-spare resilver of the first drive:

zpool replace POOL da13 da99

... but then, after that command completes and you verify that the resilver has properly begun (by running 'zpool status') you simply run a second 'zpool replace' command with the other pair of failing/spare drives:

zpool replace POOL da15 da100

Your 'zpool status' output will then show two drives resilvering with two different hot-spares and your time to completion will not increase much as compared to when you were only resilvering one drive.