The sewer main receives heated water from laundry and shower in large volumes for short periods of time, and mixes that water with contaminated sewage waste and harsh cleaning chemicals, to gravity flow to the septic system or community sewer. That large volume gravity flowing down, requires a large unobstructed vent flow to prevent it from creating a vacuum in the line. If you install 2 90 degree bends, you will do just that. You'll cause yourself problems. That heated water and chemical mix also needs to vent as straight upward (through the roof) as possible also, and the 2 -90 degree bends will cause the vented mixture to condensate at the bends and also cause the vent to fail. Contact a Licensed plumber in your area for a professional opinion.
: i am removing a bathroom to enlarge my kitchen. there is a sewer vent going up thru a wall i removed that is still being used. Can i put a 90 degree bend on the pipe where it comes out of the floor so i can run the pipe to the house wall then another 90 degree bend to go up the house wall and out thru the roof ? (the reason i may do this is that i am raising the floor 6 inches so the pipe will be able to run under the floor. the floor that is there now is a cement slab.)