do them both at the same time.

you need to squeeze the piston back into the caliper anyway to change
the pads, so it's the ideal opportunity to change the fluid if you
squeeze the pads with the bleed valve open.  that way, you're not
forcing old fluid back up into the master cylinder and you're ensuring
maximum volume change in the caliper.

I'd flush the brake fluid so when you push back the pistons, you are
pushing clean fluid back.

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