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At least they headed off the one beam, which is more than I see most plumbing contractors do. What they did, however, did substantially weaken the floor.
When the sisters are not full length, the nailing pattern becomes especially important. It's the difference between adding strength and stiffness to the joists or just adding dead weight.
The sistered 2x6s will work for strength to span 12', but they are not even close to working for deflection for a ceramic tile floor for deflection.