if there both 2gigs and are 550tis shouldnt be a problem.
also theyres prob better options than sliing 550s.
also whats ur full pc specs?
They are not both 2Gb550ti's, his is he 1Gb reference design. They do both have the same architecture though.
Asus sabertooth Z77 (x64 based)
Intel I5 3750k currently running 4x4.2Ghz
8Gb Corsair Vengence RAM 8/8/8/24