Its not exactly clear what you are trying to do but a couple of tips:
1. When you attach, you have to provide an existing pool device from which to attach
the new device, which is what the error is describing, here:
2. To me, if you want to replace the existing smaller LUNs with a larger LUN and continue
with a mirrored configuration, you'll need two larger LUNs. If you want to replace the two
smaller LUNs with one larger LUN, then that would be doable, but you would no longer
have a mirrored configuration.