Tankless water heater venting
There is no real difference from 2ea 45 degree bends, and 1ea 90 degree bend so there should be no problems in doing this. I would check the installation instructions as they may specify the total angle allowed. (I am guessing since you show 2 pipes it is a high efficiency unit with a closed fire box 1 for the fresh air and 1 for exhaust. On the large units I have installed the vents were powered by a fan and it allowed for a total of 360 degrees (4 90's)  and the pipe could run horizontal with a slight slope for condensate to drain back. on the smaller units that use natural convection they were limited to 180 with no horizontal runs. More info on the type we could provide better answers.