Can I connect two condensate pumps to the same drain line using a T connector?
The location you suggest is going to be the best suited. The U-bend is effectively a vapor/odor trap to prevent back-smell from getting through the tubing into your household. Any other location would require that you create a new U-bend to prevent those gases from returning via the new drain line. One could expect odors from one drain line returning via the other, but there's nothing to prevent that with the existing line running solo, so that would mean you don't have odors from the condensate. Ensure that the flexible tubing will not kink when connected to the T fitting and you're good to go.