why a double pump Robin, (proper term twin pull), or do you have two zones? two seperate pumps?
they dont have to be Grundfoss these days (too late , you said you had purchased) plenty of good and much cheaper ones out there. there are two types of domestic circulator in general, these are 130mm accros the flat jointing flanges or 180mm, less common, usually either side of the pump you will have isolator valves, so very little water is lost (use old towels to catch it) in the box with your new pump should be a pair of paper gaskets and a pair of rubber ditto, disconnect the elctrics, (wall end) close the valves, undo the 1.25" unions, remove pump, install new pump with rubber type if poss, and enough room, or paper type but add a little grease to them, do up unions, reconect electrics, job done. A very easy DIY job that might only be limited by the factor that you need a 18" stilson, slip wrench or big spanner to undo the unions, a lot of diy'ers just dont have that stuff.
CLEM